perl(1)
NAME
perl - Practical Extraction and Report Language
SYNOPSIS
perl [options] filename args
DESCRIPTION
Perl is an interpreted language optimized for scanning arbitrary text
files, extracting information from those text files, and printing reports
based on that information. It's also a good language for many system
management tasks. The language is intended to be practical (easy to use,
efficient, complete) rather than beautiful (tiny, elegant, minimal). It
combines (in the author's opinion, anyway) some of the best features of
C, sed, awk, and sh, so people familiar with those languages should have
little difficulty with it. (Language historians will also note some
vestiges of csh, Pascal, and even BASIC-PLUS.) Expression syntax
corresponds quite closely to C expression syntax. Unlike most Unix
utilities, perl does not arbitrarily limit the size of your data----if
you've got the memory, perl can slurp in your whole file as a single
string. Recursion is of unlimited depth. And the hash tables used by
associative arrays grow as necessary to prevent degraded performance.
Perl uses sophisticated pattern matching techniques to scan large amounts
of data very quickly. Although optimized for scanning text, perl can
also deal with binary data, and can make dbm files look like associative
arrays (where dbm is available). Setuid perl scripts are safer than C
programs through a dataflow tracing mechanism which prevents many stupid
security holes. If you have a problem that would ordinarily use sed or
awk or sh, but it exceeds their capabilities or must run a little faster,
and you don't want to write the silly thing in C, then perl may be for
you. There are also translators to turn your sed and awk scripts into
perl scripts. OK, enough hype.
Upon startup, perl looks for your script in one of the following places:
1. Specified line by line via -e switches on the command line.
2. Contained in the file specified by the first filename on the command
line. (Note that systems supporting the #! notation invoke
interpreters this way.)
3. Passed in implicitly via standard input. This only works if there
are no filename arguments----to pass arguments to a stdin script you
must explicitly specify a - for the script name.
After locating your script, perl compiles it to an internal form. If the
script is syntactically correct, it is executed.
Options
Note: on first reading this section may not make much sense to you. It's
here at the front for easy reference.
A single-character option may be combined with the following option, if
any. This is particularly useful when invoking a script using the #!
construct which only allows one argument. Example:
#!/usr/bin/perl -spi.bak # same as -s -p -i.bak
...
Options include:
-0digits
specifies the record separator ($/) as an octal number. If there
are no digits, the null character is the separator. Other switches
may precede or follow the digits. For example, if you have a
version of find which can print filenames terminated by the null
character, you can say this:
find . -name '*.bak' -print0 | perl -n0e unlink
The special value 00 will cause Perl to slurp files in paragraph
mode. The value 0777 will cause Perl to slurp files whole since
there is no legal character with that value.
-a turns on autosplit mode when used with a -n or -p. An implicit
split command to the @F array is done as the first thing inside the
implicit while loop produced by the -n or -p.
perl -ane 'print pop(@F), "\n";'
is equivalent to
while (<>) {
@F = split(' ');
print pop(@F), "\n";
}
-c causes perl to check the syntax of the script and then exit without
executing it.
-d runs the script under the perl debugger. See the section on
Debugging.
-Dnumber
sets debugging flags. To watch how it executes your script, use
-D14. (This only works if debugging is compiled into your perl.)
Another nice value is -D1024, which lists your compiled syntax tree.
And -D512 displays compiled regular expressions.
-e commandline
may be used to enter one line of script. Multiple -e commands may
be given to build up a multi-line script. If -e is given, perl will
not look for a script filename in the argument list.
-iextension
specifies that files processed by the <> construct are to be edited
in-place. It does this by renaming the input file, opening the
output file by the same name, and selecting that output file as the
default for print statements. The extension, if supplied, is added
to the name of the old file to make a backup copy. If no extension
is supplied, no backup is made. Saying ``perl -p -i.bak -e
"s/foo/bar/;" ... '' is the same as using the script:
#!/usr/bin/perl -pi.bak
s/foo/bar/;
which is equivalent to
#!/usr/bin/perl
while (<>) {
if ($ARGV ne $oldargv) {
rename($ARGV, $ARGV . '.bak');
open(ARGVOUT, ">$ARGV");
select(ARGVOUT);
$oldargv = $ARGV;
}
s/foo/bar/;
}
continue {
print; # this prints to original filename
}
select(STDOUT);
except that the -i form doesn't need to compare $ARGV to $oldargv to
know when the filename has changed. It does, however, use ARGVOUT
for the selected filehandle. Note that STDOUT is restored as the
default output filehandle after the loop.
You can use eof to locate the end of each input file, in case you
want to append to each file, or reset line numbering (see example
under eof).
-Idirectory
may be used in conjunction with -P to tell the C preprocessor where
to look for include files. By default /usr/include and
/usr/lib/perl are searched.
-loctnum
enables automatic line-ending processing. It has two effects:
first, it automatically chops the line terminator when used with -n
or -p , and second, it assigns $\ to have the value of octnum so
that any print statements will have that line terminator added back
on. If octnum is omitted, sets $\ to the current value of $/. For
instance, to trim lines to 80 columns:
perl -lpe 'substr($_, 80) = ""'
Note that the assignment $\ = $/ is done when the switch is
processed, so the input record separator can be different than the
output record separator if the -l switch is followed by a -0 switch:
gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
This sets $\ to newline and then sets $/ to the null character.
-n causes perl to assume the following loop around your script, which
makes it iterate over filename arguments somewhat like ``sed -n'' or
awk:
while (<>) {
... # your script goes here
}
Note that the lines are not printed by default. See -p to have
lines printed. Here is an efficient way to delete all files older
than a week:
find . -mtime +7 -print | perl -nle 'unlink;'
This is faster than using the -exec switch of find because you don't
have to start a process on every filename found.
-p causes perl to assume the following loop around your script, which
makes it iterate over filename arguments somewhat like sed:
while (<>) {
... # your script goes here
} continue {
print;
}
Note that the lines are printed automatically. To suppress printing
use the -n switch. A -p overrides a -n switch.
-P causes your script to be run through the C preprocessor before
compilation by perl. (Since both comments and cpp directives begin
with the # character, you should avoid starting comments with any
words recognized by the C preprocessor such as ``if'', ``else'' or
``define''.)
-s enables some rudimentary switch parsing for switches on the command
line after the script name but before any filename arguments (or
before a --). Any switch found there is removed from @ARGV and sets
the corresponding variable in the perl script. The following script
prints ``true'' if and only if the script is invoked with a -xyz
switch.
#!/usr/bin/perl -s
if ($xyz) { print "true\n"; }
-S makes perl use the PATH environment variable to search for the
script (unless the name of the script starts with a slash).
Typically this is used to emulate #! startup on machines that don't
support #!, in the following manner:
#!/usr/bin/perl
eval "exec /usr/bin/perl -S $0 $*"
if $running_under_some_shell;
The system ignores the first line and feeds the script to /bin/sh,
which proceeds to try to execute the perl script as a shell script.
The shell executes the second line as a normal shell command, and
thus starts up the perl interpreter. On some systems $0 doesn't
always contain the full pathname, so the -S tells perl to search for
the script if necessary. After perl locates the script, it parses
the lines and ignores them because the variable
$running_under_some_shell is never true. A better construct than $*
would be ${1+"$@"}, which handles embedded spaces and such in the
filenames, but doesn't work if the script is being interpreted by
csh. In order to start up sh rather than csh, some systems may have
to replace the #! line with a line containing just a colon, which
will be politely ignored by perl. Other systems can't control that,
and need a totally devious construct that will work under any of
csh, sh or perl, such as the following:
eval '(exit $?0)' && eval 'exec /usr/bin/perl -S $0
${1+"$@"}'
& eval 'exec /usr/bin/perl -S $0 $argv:q'
if 0;
-u causes perl to dump core after compiling your script. You can then
take this core dump and turn it into an executable file by using the
undump program (not supplied). This speeds startup at the expense
of some disk space (which you can minimize by stripping the
executable). (Still, a "hello world" executable comes out to about
200K on my machine.) If you are going to run your executable as a
set-id program then you should probably compile it using taintperl
rather than normal perl. If you want to execute a portion of your
script before dumping, use the dump operator instead. Note:
availability of undump is platform specific and may not be available
for a specific port of perl.
-U allows perl to do unsafe operations. Currently the only ``unsafe''
operations are the unlinking of directories while running as
superuser, and running setuid programs with fatal taint checks
turned into warnings.
-v prints the version and patchlevel of your perl executable.
-w prints warnings about identifiers that are mentioned only once, and
scalar variables that are used before being set. Also warns about
redefined subroutines, and references to undefined filehandles or
filehandles opened readonly that you are attempting to write on.
Also warns you if you use == on values that don't look like numbers,
and if your subroutines recurse more than 100 deep.
-xdirectory
tells perl that the script is embedded in a message. Leading
garbage will be discarded until the first line that starts with #!
and contains the string "perl". Any meaningful switches on that
line will be applied (but only one group of switches, as with normal
#! processing). If a directory name is specified, Perl will switch
to that directory before running the script. The -x switch only
controls the the disposal of leading garbage. The script must be
terminated with __END__ if there is trailing garbage to be ignored
(the script can process any or all of the trailing garbage via the
DATA filehandle if desired).
Data Types and Objects
Perl has three data types: scalars, arrays of scalars, and associative
arrays of scalars. Normal arrays are indexed by number, and associative
arrays by string.
The interpretation of operations and values in perl sometimes depends on
the requirements of the context around the operation or value. There are
three major contexts: string, numeric and array. Certain operations
return array values in contexts wanting an array, and scalar values
otherwise. (If this is true of an operation it will be mentioned in the
documentation for that operation.) Operations which return scalars don't
care whether the context is looking for a string or a number, but scalar
variables and values are interpreted as strings or numbers as appropriate
to the context. A scalar is interpreted as TRUE in the boolean sense if
it is not the null string or 0. Booleans returned by operators are 1 for
true and 0 or '' (the null string) for false.
There are actually two varieties of null string: defined and undefined.
Undefined null strings are returned when there is no real value for
something, such as when there was an error, or at end of file, or when
you refer to an uninitialized variable or element of an array. An
undefined null string may become defined the first time you access it,
but prior to that you can use the defined() operator to determine whether
the value is defined or not.
References to scalar variables always begin with `$', even when referring
to a scalar that is part of an array. Thus:
$days # a simple scalar variable
$days[28] # 29th element of array @days
$days{'Feb'} # one value from an associative array
$#days # last index of array @days
but entire arrays or array slices are denoted by `@':
@days # ($days[0], $days[1],... $days[n])
@days[3,4,5] # same as @days[3..5]
@days{'a','c'} # same as ($days{'a'},$days{'c'})
and entire associative arrays are denoted by `%':
%days # (key1, val1, key2, val2 ...)
Any of these eight constructs may serve as an lvalue, that is, may be
assigned to. (It also turns out that an assignment is itself an lvalue
in certain contexts----see examples under s, tr and chop.) Assignment to
a scalar evaluates the righthand side in a scalar context, while
assignment to an array or array slice evaluates the righthand side in an
array context.
You may find the length of array @days by evaluating ``$#days'', as in
csh. (Actually, it's not the length of the array, it's the subscript of
the last element, since there is (ordinarily) a 0th element.) Assigning
to $#days changes the length of the array. Shortening an array by this
method does not actually destroy any values. Lengthening an array that
was previously shortened recovers the values that were in those elements.
You can also gain some measure of efficiency by preextending an array
that is going to get big. (You can also extend an array by assigning to
an element that is off the end of the array. This differs from assigning
to $#whatever in that intervening values are set to null rather than
recovered.) You can truncate an array down to nothing by assigning the
null list () to it. The following are exactly equivalent
@whatever = ();
$#whatever = $[ - 1;
If you evaluate an array in a scalar context, it returns the length of
the array. The following is always true:
scalar(@whatever) == $#whatever - $[ + 1;
If you evaluate an associative array in a scalar context, it returns a
value which is true if and only if the array contains any elements. (If
there are any elements, the value returned is a string consisting of the
number of used buckets and the number of allocated buckets, separated by
a slash.)
Multi-dimensional arrays are not directly supported, but see the
discussion of the $; variable later for a means of emulating multiple
subscripts with an associative array. You could also write a subroutine
to turn multiple subscripts into a single subscript.
Every data type has its own namespace. You can, without fear of
conflict, use the same name for a scalar variable, an array, an
associative array, a filehandle, a subroutine name, and/or a label.
Since variable and array references always start with `$', `@', or `%',
the ``reserved'' words aren't in fact reserved with respect to variable
names. (They ARE reserved with respect to labels and filehandles,
however, which don't have an initial special character. Hint: you could
say open(LOG,'logfile') rather than open(log,'logfile'). Using uppercase
filehandles also improves readability and protects you from conflict with
future reserved words.) Case IS significant----``FOO'', ``Foo'' and
``foo'' are all different names. Names which start with a letter may
also contain digits and underscores. Names which do not start with a
letter are limited to one character, e.g. ``$%'' or ``$$''. (Most of the
one character names have a predefined significance to perl. More later.)
Numeric literals are specified in any of the usual floating point or
integer formats:
12345
12345.67
.23E-10
0xffff # hex
0377 # octal
4_294_967_296
String literals are delimited by either single or double quotes. They
work much like shell quotes: double-quoted string literals are subject
to backslash and variable substitution; single-quoted strings are not
(except for \' and \\). The usual backslash rules apply for making
characters such as newline, tab, etc., as well as some more exotic forms:
\t tab
\n newline
\r return
\f form feed
\b backspace
\a alarm (bell)
\e escape
\033 octal char
\x1b hex char
\c[ control char
\l lowercase next char
\u uppercase next char
\L lowercase till \E
\U uppercase till \E
\E end case modification
You can also embed newlines directly in your strings, i.e. they can end
on a different line than they begin. This is nice, but if you forget
your trailing quote, the error will not be reported until perl finds
another line containing the quote character, which may be much further on
in the script. Variable substitution inside strings is limited to scalar
variables, normal array values, and array slices. (In other words,
identifiers beginning with $ or @, followed by an optional bracketed
expression as a subscript.) The following code segment prints out ``The
price is $100.''
$Price = '$100'; # not interpreted
print "The price is $Price.\n"; # interpreted
Note that you can put curly brackets around the identifier to delimit it
from following alphanumerics. Also note that a single quoted string must
be separated from a preceding word by a space, since single quote is a
valid character in an identifier (see Packages).
Two special literals are __LINE__ and __FILE__, which represent the
current line number and filename at that point in your program. They may
only be used as separate tokens; they will not be interpolated into
strings. In addition, the token __END__ may be used to indicate the
logical end of the script before the actual end of file. Any following
text is ignored, but may be read via the DATA filehandle. (The DATA
filehandle may read data only from the main script, but not from any
required file or evaluated string.) The two control characters ^D and ^Z
are synonyms for __END__.
A word that doesn't have any other interpretation in the grammar will be
treated as if it had single quotes around it. For this purpose, a word
consists only of alphanumeric characters and underline, and must start
with an alphabetic character. As with filehandles and labels, a bare
word that consists entirely of lowercase letters risks conflict with
future reserved words, and if you use the -w switch, Perl will warn you
about any such words.
Array values are interpolated into double-quoted strings by joining all
the elements of the array with the delimiter specified in the $"
variable, space by default. (Since in versions of perl prior to 3.0 the
@ character was not a metacharacter in double-quoted strings, the
interpolation of @array, $array[EXPR], @array[LIST], $array{EXPR}, or
@array{LIST} only happens if array is referenced elsewhere in the program
or is predefined.) The following are equivalent:
$temp = join($",@ARGV);
system "echo $temp";
system "echo @ARGV";
Within search patterns (which also undergo double-quotish substitution)
there is a bad ambiguity: Is /$foo[bar]/ to be interpreted as
/${foo}[bar]/ (where [bar] is a character class for the regular
expression) or as /${foo[bar]}/ (where [bar] is the subscript to array
@foo)? If @foo doesn't otherwise exist, then it's obviously a character
class. If @foo exists, perl takes a good guess about [bar], and is
almost always right. If it does guess wrong, or if you're just plain
paranoid, you can force the correct interpretation with curly brackets as
above.
A line-oriented form of quoting is based on the shell here-is syntax.
Following a << you specify a string to terminate the quoted material, and
all lines following the current line down to the terminating string are
the value of the item. The terminating string may be either an
identifier (a word), or some quoted text. If quoted, the type of quotes
you use determines the treatment of the text, just as in regular quoting.
An unquoted identifier works like double quotes. There must be no space
between the << and the identifier. (If you put a space it will be
treated as a null identifier, which is valid, and matches the first blank
line----see Merry Christmas example below.) The terminating string must
appear by itself (unquoted and with no surrounding whitespace) on the
terminating line.
print <<EOF; # same as above
The price is $Price.
EOF
print <<"EOF"; # same as above
The price is $Price.
EOF
print << x 10; # null identifier is delimiter
Merry Christmas!
print <<`EOC`; # execute commands
echo hi there
echo lo there
EOC
print <<foo, <<bar; # you can stack them
I said foo.
foo
I said bar.
bar
Array literals are denoted by separating individual values by commas, and
enclosing the list in parentheses:
(LIST)
In a context not requiring an array value, the value of the array literal
is the value of the final element, as in the C comma operator. For
example,
@foo = ('cc', '-E', $bar);
assigns the entire array value to array foo, but
$foo = ('cc', '-E', $bar);
assigns the value of variable bar to variable foo. Note that the value
of an actual array in a scalar context is the length of the array; the
following assigns to $foo the value 3:
@foo = ('cc', '-E', $bar);
$foo = @foo; # $foo gets 3
You may have an optional comma before the closing parenthesis of an array
literal, so that you can say:
@foo = (
1,
2,
3,
);
When a LIST is evaluated, each element of the list is evaluated in an
array context, and the resulting array value is interpolated into LIST
just as if each individual element were a member of LIST. Thus arrays
lose their identity in a LIST----the list
(@foo,@bar,&SomeSub)
contains all the elements of @foo followed by all the elements of @bar,
followed by all the elements returned by the subroutine named SomeSub.
A list value may also be subscripted like a normal array. Examples:
$time = (stat($file))[8]; # stat returns array value
$digit = ('a','b','c','d','e','f')[$digit-10];
return (pop(@foo),pop(@foo))[0];
Array lists may be assigned to if and only if each element of the list is
an lvalue:
($a, $b, $c) = (1, 2, 3);
($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
The final element may be an array or an associative array:
($a, $b, @rest) = split;
local($a, $b, %rest) = @_;
You can actually put an array anywhere in the list, but the first array
in the list will soak up all the values, and anything after it will get a
null value. This may be useful in a local().
An associative array literal contains pairs of values to be interpreted
as a key and a value:
# same as map assignment above
%map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
Array assignment in a scalar context returns the number of elements
produced by the expression on the right side of the assignment:
$x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
There are several other pseudo-literals that you should know about. If a
string is enclosed by backticks (grave accents), it first undergoes
variable substitution just like a double quoted string. It is then
interpreted as a command, and the output of that command is the value of
the pseudo-literal, like in a shell. In a scalar context, a single
string consisting of all the output is returned. In an array context, an
array of values is returned, one for each line of output. (You can set
$/ to use a different line terminator.) The command is executed each
time the pseudo-literal is evaluated. The status value of the command is
returned in $? (see Predefined Names for the interpretation of $?).
Unlike in csh, no translation is done on the return data----newlines
remain newlines. Unlike in any of the shells, single quotes do not hide
variable names in the command from interpretation. To pass a $ through
to the shell you need to hide it with a backslash.
Evaluating a filehandle in angle brackets yields the next line from that
file (newline included, so it's never false until EOF, at which time an
undefined value is returned). Ordinarily you must assign that value to a
variable, but there is one situation where an automatic assignment
happens. If (and only if) the input symbol is the only thing inside the
conditional of a while loop, the value is automatically assigned to the
variable ``$_''. (This may seem like an odd thing to you, but you'll use
the construct in almost every perl script you write.) Anyway, the
following lines are equivalent to each other:
while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while $_ = <STDIN>;
print while <STDIN>;
The filehandles STDIN, STDOUT and STDERR are predefined. (The
filehandles stdin, stdout and stderr will also work except in packages,
where they would be interpreted as local identifiers rather than global.)
Additional filehandles may be created with the open function.
If a <FILEHANDLE> is used in a context that is looking for an array, an
array consisting of all the input lines is returned, one line per array
element. It's easy to make a LARGE data space this way, so use with
care.
The null filehandle <> is special and can be used to emulate the behavior
of sed and awk. Input from <> comes either from standard input, or from
each file listed on the command line. Here's how it works: the first
time <> is evaluated, the ARGV array is checked, and if it is null,
$ARGV[0] is set to '-', which when opened gives you standard input. The
ARGV array is then processed as a list of filenames. The loop
while (<>) {
... # code for each line
}
is equivalent to the following Perl-like pseudo code:
unshift(@ARGV, '-') if $#ARGV < $[;
while ($ARGV = shift) {
open(ARGV, $ARGV);
while (<ARGV>) {
... # code for each line
}
}
except that it isn't as cumbersome to say, and will actually work. It
really does shift array ARGV and put the current filename into variable
ARGV. It also uses filehandle ARGV internally----<> is just a synonym
for <ARGV>, which is magical. (The pseudo code above doesn't work
because it treats <ARGV> as non-magical.)
You can modify @ARGV before the first <> as long as the array ends up
containing the list of filenames you really want. Line numbers ($.)
continue as if the input was one big happy file. (But see example under
eof for how to reset line numbers on each file.)
If you want to set @ARGV to your own list of files, go right ahead. If
you want to pass switches into your script, you can put a loop on the
front like this:
while ($_ = $ARGV[0], /^-/) {
shift;
last if /^--$/;
/^-D(.*)/ && ($debug = $1);
/^-v/ && $verbose++;
... # other switches
}
while (<>) {
... # code for each line
}
The <> symbol will return FALSE only once. If you call it again after
this it will assume you are processing another @ARGV list, and if you
haven't set @ARGV, will input from STDIN.
If the string inside the angle brackets is a reference to a scalar
variable (e.g. <$foo>), then that variable contains the name of the
filehandle to input from.
If the string inside angle brackets is not a filehandle, it is
interpreted as a filename pattern to be globbed, and either an array of
filenames or the next filename in the list is returned, depending on
context. One level of $ interpretation is done first, but you can't say
<$foo> because that's an indirect filehandle as explained in the previous
paragraph. You could insert curly brackets to force interpretation as a
filename glob: <${foo}>. Example:
while (<*.c>) {
chmod 0644, $_;
}
is equivalent to
open(foo, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<foo>) {
chop;
chmod 0644, $_;
}
In fact, it's currently implemented that way. (Which means it will not
work on filenames with spaces in them unless you have /bin/csh on your
machine.) Of course, the shortest way to do the above is:
chmod 0644, <*.c>;
Syntax
A perl script consists of a sequence of declarations and commands. The
only things that need to be declared in perl are report formats and
subroutines. See the sections below for more information on those
declarations. All uninitialized user-created objects are assumed to
start with a null or 0 value until they are defined by some explicit
operation such as assignment. The sequence of commands is executed just
once, unlike in sed and awk scripts, where the sequence of commands is
executed for each input line. While this means that you must explicitly
loop over the lines of your input file (or files), it also means you have
much more control over which files and which lines you look at.
(Actually, I'm lying----it is possible to do an implicit loop with either
the -n or -p switch.)
A declaration can be put anywhere a command can, but has no effect on the
execution of the primary sequence of commands----declarations all take
effect at compile time. Typically all the declarations are put at the
beginning or the end of the script.
Perl is, for the most part, a free-form language. (The only exception to
this is format declarations, for fairly obvious reasons.) Comments are
indicated by the # character, and extend to the end of the line. If you
attempt to use /* */ C comments, it will be interpreted either as
division or pattern matching, depending on the context. So don't do
that.
Compound statements
In perl, a sequence of commands may be treated as one command by
enclosing it in curly brackets. We will call this a BLOCK.
The following compound commands may be used to control flow:
if (EXPR) BLOCK
if (EXPR) BLOCK else BLOCK
if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
LABEL while (EXPR) BLOCK
LABEL while (EXPR) BLOCK continue BLOCK
LABEL for (EXPR; EXPR; EXPR) BLOCK
LABEL foreach VAR (ARRAY) BLOCK
LABEL BLOCK continue BLOCK
Note that, unlike C and Pascal, these are defined in terms of BLOCKs, not
statements. This means that the curly brackets are required----no
dangling statements allowed. If you want to write conditionals without
curly brackets there are several other ways to do it. The following all
do the same thing:
if (!open(foo)) { die "Can't open $foo: $!"; }
die "Can't open $foo: $!" unless open(foo);
open(foo) || die "Can't open $foo: $!"; # foo or bust!
open(foo) ? 'hi mom' : die "Can't open $foo: $!";
# a bit exotic, that last one
The if statement is straightforward. Since BLOCKs are always bounded by
curly brackets, there is never any ambiguity about which if an else goes
with. If you use unless in place of if, the sense of the test is
reversed.
The while statement executes the block as long as the expression is true
(does not evaluate to the null string or 0). The LABEL is optional, and
if present, consists of an identifier followed by a colon. The LABEL
identifies the loop for the loop control statements next, last, and redo
(see below). If there is a continue BLOCK, it is always executed just
before the conditional is about to be evaluated again, similarly to the
third part of a for loop in C. Thus it can be used to increment a loop
variable, even when the loop has been continued via the next statement
(similar to the C ``continue'' statement).
If the word while is replaced by the word until, the sense of the test is
reversed, but the conditional is still tested before the first iteration.
In either the if or the while statement, you may replace ``(EXPR)'' with
a BLOCK, and the conditional is true if the value of the last command in
that block is true.
The for loop works exactly like the corresponding while loop:
for ($i = 1; $i < 10; $i++) {
...
}
is the same as
$i = 1;
while ($i < 10) {
...
} continue {
$i++;
}
The foreach loop iterates over a normal array value and sets the variable
VAR to be each element of the array in turn. The variable is implicitly
local to the loop, and regains its former value upon exiting the loop.
The ``foreach'' keyword is actually identical to the ``for'' keyword, so
you can use ``foreach'' for readability or ``for'' for brevity. If VAR
is omitted, $_ is set to each value. If ARRAY is an actual array (as
opposed to an expression returning an array value), you can modify each
element of the array by modifying VAR inside the loop. Examples:
for (@ary) { s/foo/bar/; }
foreach $elem (@elements) {
$elem *= 2;
}
for ((10,9,8,7,6,5,4,3,2,1,'BOOM')) {
print $_, "\n"; sleep(1);
}
for (1..15) { print "Merry Christmas\n"; }
foreach $item (split(/:[\\\n:]*/, $ENV{'TERMCAP'})) {
print "Item: $item\n";
}
The BLOCK by itself (labeled or not) is equivalent to a loop that
executes once. Thus you can use any of the loop control statements in it
to leave or restart the block. The continue block is optional. This
construct is particularly nice for doing case structures.
foo: {
if (/^abc/) { $abc = 1; last foo; }
if (/^def/) { $def = 1; last foo; }
if (/^xyz/) { $xyz = 1; last foo; }
$nothing = 1;
}
There is no official switch statement in perl, because there are already
several ways to write the equivalent. In addition to the above, you
could write
foo: {
$abc = 1, last foo if /^abc/;
$def = 1, last foo if /^def/;
$xyz = 1, last foo if /^xyz/;
$nothing = 1;
}
or
foo: {
/^abc/ && do { $abc = 1; last foo; };
/^def/ && do { $def = 1; last foo; };
/^xyz/ && do { $xyz = 1; last foo; };
$nothing = 1;
}
or
foo: {
/^abc/ && ($abc = 1, last foo);
/^def/ && ($def = 1, last foo);
/^xyz/ && ($xyz = 1, last foo);
$nothing = 1;
}
or even
if (/^abc/)
{ $abc = 1; }
elsif (/^def/)
{ $def = 1; }
elsif (/^xyz/)
{ $xyz = 1; }
else
{$nothing = 1;}
As it happens, these are all optimized internally to a switch structure,
so perl jumps directly to the desired statement, and you needn't worry
about perl executing a lot of unnecessary statements when you have a
string of 50 elsifs, as long as you are testing the same simple scalar
variable using ==, eq, or pattern matching as above. (If you're curious
as to whether the optimizer has done this for a particular case
statement, you can use the -D1024 switch to list the syntax tree before
execution.)
Simple statements
The only kind of simple statement is an expression evaluated for its side
effects. Every simple statement must be terminated with a semicolon,
unless it is the final statement in a block, in which case the semicolon
is optional. (Semicolon is still encouraged there if the block takes up
more than one line).
Any simple statement may optionally be followed by a single modifier,
just before the terminating semicolon. The possible modifiers are:
if EXPR
unless EXPR
while EXPR
until EXPR
The if and unless modifiers have the expected semantics. The while and
until modifiers also have the expected semantics (conditional evaluated
first), except when applied to a do-BLOCK or a do-SUBROUTINE command, in
which case the block executes once before the conditional is evaluated.
This is so that you can write loops like:
do {
$_ = <STDIN>;
...
} until $_ eq ".\n";
(See the do operator below. Note also that the loop control commands
described later will NOT work in this construct, since modifiers don't
take loop labels. Sorry.)
Expressions
Since perl expressions work almost exactly like C expressions, only the
differences will be mentioned here.
Here's what perl has that C doesn't:
** The exponentiation operator.
**= The exponentiation assignment operator.
() The null list, used to initialize an array to null.
. Concatenation of two strings.
.= The concatenation assignment operator.
eq String equality (== is numeric equality). For a mnemonic just
think of ``eq'' as a string. (If you are used to the awk
behavior of using == for either string or numeric equality based
on the current form of the comparands, beware! You must be
explicit here.)
ne String inequality (!= is numeric inequality).
lt String less than.
gt String greater than.
le String less than or equal.
ge String greater than or equal.
cmp String comparison, returning -1, 0, or 1.
<=> Numeric comparison, returning -1, 0, or 1.
=~ Certain operations search or modify the string ``$_'' by default.
This operator makes that kind of operation work on some other
string. The right argument is a search pattern, substitution, or
translation. The left argument is what is supposed to be
searched, substituted, or translated instead of the default
``$_''. The return value indicates the success of the operation.
(If the right argument is an expression other than a search
pattern, substitution, or translation, it is interpreted as a
search pattern at run time. This is less efficient than an
explicit search, since the pattern must be compiled every time
the expression is evaluated.) The precedence of this operator is
lower than unary minus and autoincrement/decrement, but higher
than everything else.
!~ Just like =~ except the return value is negated.
x The repetition operator. Returns a string consisting of the left
operand repeated the number of times specified by the right
operand. In an array context, if the left operand is a list in
parens, it repeats the list.
print '-' x 80; # print row of dashes
print '-' x80; # illegal, x80 is identifier
print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
@ones = (1) x 80; # an array of 80 1's
@ones = (5) x @ones; # set all elements to 5
x= The repetition assignment operator. Only works on scalars.
.. The range operator, which is really two different operators
depending on the context. In an array context, returns an array
of values counting (by ones) from the left value to the right
value. This is useful for writing ``for (1..10)'' loops and for
doing slice operations on arrays.
In a scalar context, .. returns a boolean value. The operator is
bistable, like a flip-flop, and emulates the line-range (comma)
operator of sed, awk, and various editors. Each .. operator
maintains its own boolean state. It is false as long as its left
operand is false. Once the left operand is true, the range
operator stays true until the right operand is true, AFTER which
the range operator becomes false again. (It doesn't become false
till the next time the range operator is evaluated. It can test
the right operand and become false on the same evaluation it
became true (as in awk), but it still returns true once. If you
don't want it to test the right operand till the next evaluation
(as in sed), use three dots (...) instead of two.) The right
operand is not evaluated while the operator is in the ``false''
state, and the left operand is not evaluated while the operator
is in the ``true'' state. The precedence is a little lower than
|| and &&. The value returned is either the null string for
false, or a sequence number (beginning with 1) for true. The
sequence number is reset for each range encountered. The final
sequence number in a range has the string 'E0' appended to it,
which doesn't affect its numeric value, but gives you something
to search for if you want to exclude the endpoint. You can
exclude the beginning point by waiting for the sequence number to
be greater than 1. If either operand of scalar .. is static,
that operand is implicitly compared to the $. variable, the
current line number. Examples:
As a scalar operator:
if (101 .. 200) { print; } # print 2nd hundred lines
next line if (1 .. /^$/); # skip header lines
s/^/> / if (/^$/ .. eof()); # quote body
As an array operator:
for (101 .. 200) { print; } # print $_ 100 times
@foo = @foo[$[ .. $#foo]; # an expensive no-op
@foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
-x A file test. This unary operator takes one argument, either a
filename or a filehandle, and tests the associated file to see if
something is true about it. If the argument is omitted, tests
$_, except for -t, which tests STDIN. It returns 1 for true and
'' for false, or the undefined value if the file doesn't exist.
Precedence is higher than logical and relational operators, but
lower than arithmetic operators. The operator may be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size.
-s File has non-zero size (returns size).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO).
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-t Filehandle is opened to a tty.
-T File is a text file.
-B File is a binary file (opposite of -T).
-M Age of file in days when script started.
-A Same for access time.
-C Same for inode change time.
The interpretation of the file permission operators -r, -R, -w,
-W, -x and -X is based solely on the mode of the file and the
uids and gids of the user. There may be other reasons you can't
actually read, write or execute the file. Also note that, for
the superuser, -r, -R, -w and -W always return 1, and -x and -X
return 1 if any execute bit is set in the mode. Scripts run by
the superuser may thus need to do a stat() in order to determine
the actual mode of the file, or temporarily set the uid to
something else.
Example:
while (<>) {
chop;
next unless -f $_; # ignore specials
...
}
Note that -s/a/b/ does not do a negated substitution. Saying
-exp($foo) still works as expected, however----only single
letters following a minus are interpreted as file tests.
The -T and -B switches work as follows. The first block or so of
the file is examined for odd characters such as strange control
codes or metacharacters. If too many odd characters (>10%) are
found, it's a -B file, otherwise it's a -T file. Also, any file
containing null in the first block is considered a binary file.
If -T or -B is used on a filehandle, the current stdio buffer is
examined rather than the first block. Both -T and -B return TRUE
on a null file, or a file at EOF when testing a filehandle.
If any of the file tests (or either stat operator) are given the special
filehandle consisting of a solitary underline, then the stat structure of
the previous file test (or stat operator) is used, saving a system call.
(This doesn't work with -t, and you need to remember that lstat and -l
will leave values in the stat structure for the symbolic link, not the
real file.) Example:
print "Can do.\n" if -r $a || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
Here is what C has that perl doesn't:
unary & Address-of operator.
unary * Dereference-address operator.
(TYPE) Type casting operator.
Like C, perl does a certain amount of expression evaluation at compile
time, whenever it determines that all of the arguments to an operator are
static and have no side effects. In particular, string concatenation
happens at compile time between literals that don't do variable
substitution. Backslash interpretation also happens at compile time.
You can say
'Now is the time for all' . "\n" .
'good men to come to.'
and this all reduces to one string internally.
The autoincrement operator has a little extra built-in magic to it. If
you increment a variable that is numeric, or that has ever been used in a
numeric context, you get a normal increment. If, however, the variable
has only been used in string contexts since it was set, and has a value
that is not null and matches the pattern /^[a-zA-Z]*[0-9]*$/, the
increment is done as a string, preserving each character within its
range, with carry:
print ++($foo = '99'); # prints `100'
print ++($foo = 'a0'); # prints `a1'
print ++($foo = 'Az'); # prints `Ba'
print ++($foo = 'zz'); # prints `aaa'
The autodecrement is not magical.
The range operator (in an array context) makes use of the magical
autoincrement algorithm if the minimum and maximum are strings. You can
say
@alphabet = ('A' .. 'Z');
to get all the letters of the alphabet, or
$hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
to get a hexadecimal digit, or
@z2 = ('01' .. '31'); print @z2[$mday];
to get dates with leading zeros. (If the final value specified is not in
the sequence that the magical increment would produce, the sequence goes
until the next value would be longer than the final value specified.)
The || and && operators differ from C's in that, rather than returning 0
or 1, they return the last value evaluated. Thus, a portable way to find
out the home directory might be:
$home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
(getpwuid($<))[7] || die "You're homeless!\n";
Along with the literals and variables mentioned earlier, the operations
in the following section can serve as terms in an expression. Some of
these operations take a LIST as an argument. Such a list can consist of
any combination of scalar arguments or array values; the array values
will be included in the list as if each individual element were
interpolated at that point in the list, forming a longer single-
dimensional array value. Elements of the LIST should be separated by
commas. If an operation is listed both with and without parentheses
around its arguments, it means you can either use it as a unary operator
or as a function call. To use it as a function call, the next token on
the same line must be a left parenthesis. (There may be intervening
white space.) Such a function then has highest precedence, as you would
expect from a function. If any token other than a left parenthesis
follows, then it is a unary operator, with a precedence depending only on
whether it is a LIST operator or not. LIST operators have lowest
precedence. All other unary operators have a precedence greater than
relational operators but less than arithmetic operators. See the section
on Precedence.
For operators that can be used in either a scalar or array context,
failure is generally indicated in a scalar context by returning the
undefined value, and in an array context by returning the null list.
Remember though that THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO
A SCALAR. Each operator decides which sort of scalar it would be most
appropriate to return. Some operators return the length of the list that
would have been returned in an array context. Some operators return the
first value in the list. Some operators return the last value in the
list. Some operators return a count of successful operations. In
general, they do what you want, unless you want consistency.
/PATTERN/
See m/PATTERN/.
?PATTERN?
This is just like the /pattern/ search, except that it matches
only once between calls to the reset operator. This is a useful
optimization when you only want to see the first occurrence of
something in each file of a set of files, for instance. Only ??
patterns local to the current package are reset.
accept(NEWSOCKET,GENERICSOCKET)
Does the same thing that the accept system call does. Returns
true if it succeeded, false otherwise. See example in section on
Interprocess Communication.
alarm(SECONDS)
alarm SECONDS
Arranges to have a SIGALRM delivered to this process after the
specified number of seconds (minus 1, actually) have elapsed.
Thus, alarm(15) will cause a SIGALRM at some point more than 14
seconds in the future. Only one timer may be counting at once.
Each call disables the previous timer, and an argument of 0 may
be supplied to cancel the previous timer without starting a new
one. The returned value is the amount of time remaining on the
previous timer.
atan2(Y,X)
Returns the arctangent of Y/X in the range -PI to PI.
bind(SOCKET,NAME)
Does the same thing that the bind system call does. Returns true
if it succeeded, false otherwise. NAME should be a packed
address of the proper type for the socket. See example in
section on Interprocess Communication.
binmode(FILEHANDLE)
binmode FILEHANDLE
Arranges for the file to be read in ``binary'' mode in operating
systems that distinguish between binary and text files. Files
that are not read in binary mode have CR LF sequences translated
to LF on input and LF translated to CR LF on output. Binmode has
no effect under Unix. If FILEHANDLE is an expression, the value
is taken as the name of the filehandle.
caller(EXPR)
caller Returns the context of the current subroutine call:
($package,$filename,$line) = caller;
With EXPR, returns some extra information that the debugger uses
to print a stack trace. The value of EXPR indicates how many
call frames to go back before the current one.
chdir(EXPR)
chdir EXPR
Changes the working directory to EXPR, if possible. If EXPR is
omitted, changes to home directory. Returns 1 upon success, 0
otherwise. See example under die.
chmod(LIST)
chmod LIST
Changes the permissions of a list of files. The first element of
the list must be the numerical mode. Returns the number of files
successfully changed.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
chop(LIST)
chop(VARIABLE)
chop VARIABLE
chop Chops off the last character of a string and returns the
character chopped. It's used primarily to remove the newline
from the end of an input record, but is much more efficient than
s/\n// because it neither scans nor copies the string. If
VARIABLE is omitted, chops $_. Example:
while (<>) {
chop; # avoid \n on last field
@array = split(/:/);
...
}
You can actually chop anything that's an lvalue, including an
assignment:
chop($cwd = `pwd`);
chop($answer = <STDIN>);
If you chop a list, each element is chopped. Only the value of
the last chop is returned.
chown(LIST)
chown LIST
Changes the owner (and group) of a list of files. The first two
elements of the list must be the NUMERICAL uid and gid, in that
order. Returns the number of files successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up non-numeric uids in the passwd
file:
print "User: ";
$user = <STDIN>;
chop($user);
print "Files: "
$pattern = <STDIN>;
chop($pattern);
open(pass, '/etc/passwd')
|| die "Can't open passwd: $!\n";
while (<pass>) {
($login,$pass,$uid,$gid) = split(/:/);
$uid{$login} = $uid;
$gid{$login} = $gid;
}
@ary = <${pattern}>; # get filenames
if ($uid{$user} eq '') {
die "$user not in passwd file";
}
else {
chown $uid{$user}, $gid{$user}, @ary;
}
chroot(FILENAME)
chroot FILENAME
Does the same as the system call of that name. If you don't know
what it does, don't worry about it. If FILENAME is omitted, does
chroot to $_.
close(FILEHANDLE)
close FILEHANDLE
Closes the file or pipe associated with the file handle. You
don't have to close FILEHANDLE if you are immediately going to do
another open on it, since open will close it for you. (See
open.) However, an explicit close on an input file resets the
line counter ($.), while the implicit close done by open does
not. Also, closing a pipe will wait for the process executing on
the pipe to complete, in case you want to look at the output of
the pipe afterwards. Closing a pipe explicitly also puts the
status value of the command into $?. Example:
open(OUTPUT, '|sort >foo'); # pipe to sort
... # print stuff to output
close OUTPUT; # wait for sort to finish
open(INPUT, 'foo'); # get sort's results
FILEHANDLE may be an expression whose value gives the real
filehandle name.
closedir(DIRHANDLE)
closedir DIRHANDLE
Closes a directory opened by opendir().
connect(SOCKET,NAME)
Does the same thing that the connect system call does. Returns
true if it succeeded, false otherwise. NAME should be a package
address of the proper type for the socket. See example in
section on Interprocess Communication.
cos(EXPR)
cos EXPR
Returns the cosine of EXPR (expressed in radians). If EXPR is
omitted takes cosine of $_.
crypt(PLAINTEXT,SALT)
Encrypts a string exactly like the crypt() function in the C
library. Useful for checking the password file for lousy
passwords. Only the guys wearing white hats should do this.
dbmclose(ASSOC_ARRAY)
dbmclose ASSOC_ARRAY
Breaks the binding between a dbm file and an associative array.
The values remaining in the associative array are meaningless
unless you happen to want to know what was in the cache for the
dbm file. This function is only useful if you have ndbm.
dbmopen(ASSOC,DBNAME,MODE)
This binds a dbm or ndbm file to an associative array. ASSOC is
the name of the associative array. (Unlike normal open, the
first argument is NOT a filehandle, even though it looks like
one). DBNAME is the name of the database (without the .dir or
.pag extension). If the database does not exist, it is created
with protection specified by MODE (as modified by the umask). If
your system only supports the older dbm functions, you may
perform only one dbmopen in your program. If your system has
neither dbm nor ndbm, calling dbmopen produces a fatal error.
Values assigned to the associative array prior to the dbmopen are
lost. A certain number of values from the dbm file are cached in
memory. By default this number is 64, but you can increase it by
preallocating that number of garbage entries in the associative
array before the dbmopen. You can flush the cache if necessary
with the reset command.
If you don't have write access to the dbm file, you can only read
associative array variables, not set them. If you want to test
whether you can write, either use file tests or try setting a
dummy array entry inside an eval, which will trap the error.
Note that functions such as keys() and values() may return huge
array values when used on large dbm files. You may prefer to use
the each() function to iterate over large dbm files. Example:
# print out history file offsets
dbmopen(HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(HIST);
defined(EXPR)
defined EXPR
Returns a boolean value saying whether the lvalue EXPR has a real
value or not. Many operations return the undefined value under
exceptional conditions, such as end of file, uninitialized
variable, system error and such. This function allows you to
distinguish between an undefined null string and a defined null
string with operations that might return a real null string, in
particular referencing elements of an array. You may also check
to see if arrays or subroutines exist. Use on predefined
variables is not guaranteed to produce intuitive results.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
eval '@foo = ()' if defined(@foo);
die "No XYZ package defined" unless defined %_XYZ;
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
See also undef.
delete $ASSOC{KEY}
Deletes the specified value from the specified associative array.
Returns the deleted value, or the undefined value if nothing was
deleted. Deleting from $ENV{} modifies the environment.
Deleting from an array bound to a dbm file deletes the entry from
the dbm file.
The following deletes all the values of an associative array:
foreach $key (keys %ARRAY) {
delete $ARRAY{$key};
}
(But it would be faster to use the reset command. Saying undef
%ARRAY is faster yet.)
die(LIST)
die LIST
Outside of an eval, prints the value of LIST to STDERR and exits
with the current value of $! (errno). If $! is 0, exits with
the value of ($? >> 8) (`command` status). If ($? >> 8) is 0,
exits with 255. Inside an eval, the error message is stuffed
into $@ and the eval is terminated with the undefined value.
Equivalent examples:
die "Can't cd to spool: $!\n"
unless chdir '/usr/spool/news';
chdir '/usr/spool/news' || die "Can't cd to spool: $!\n"
If the value of EXPR does not end in a newline, the current
script line number and input line number (if any) are also
printed, and a newline is supplied. Hint: sometimes appending
``, stopped'' to your message will cause it to make better sense
when the string ``at foo line 123'' is appended. Suppose you are
running script ``canasta''.
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also exit.
do BLOCK
Returns the value of the last command in the sequence of commands
indicated by BLOCK. When modified by a loop modifier, executes
the BLOCK once before testing the loop condition. (On other
statements the loop modifiers test the conditional first.)
do SUBROUTINE (LIST)
Executes a SUBROUTINE declared by a sub declaration, and returns
the value of the last expression evaluated in SUBROUTINE. If
there is no subroutine by that name, produces a fatal error.
(You may use the ``defined'' operator to determine if a
subroutine exists.) If you pass arrays as part of LIST you may
wish to pass the length of the array in front of each array.
(See the section on subroutines later on.) The parentheses are
required to avoid confusion with the ``do EXPR'' form.
SUBROUTINE may also be a single scalar variable, in which case
the name of the subroutine to execute is taken from the variable.
As an alternate (and preferred) form, you may call a subroutine
by prefixing the name with an ampersand: &foo(@args). If you
aren't passing any arguments, you don't have to use parentheses.
If you omit the parentheses, no @_ array is passed to the
subroutine. The & form is also used to specify subroutines to
the defined and undef operators:
if (defined &$var) { &$var($parm); undef &$var; }
do EXPR Uses the value of EXPR as a filename and executes the contents of
the file as a perl script. Its primary use is to include
subroutines from a perl subroutine library.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient, more concise, keeps track of the
current filename for error messages, and searches all the -I
libraries if the file isn't in the current directory (see also
the @INC array in Predefined Names). It's the same, however, in
that it does reparse the file every time you call it, so if you
are going to use the file inside a loop you might prefer to use
-P and #include, at the expense of a little more startup time.
(The main problem with #include is that cpp doesn't grok #
comments----a workaround is to use ``;#'' for standalone
comments.) Note that the following are NOT equivalent:
do $foo; # eval a file
do $foo(); # call a subroutine
Note that inclusion of library routines is better done with the
``require'' operator.
dump LABEL
This causes an immediate core dump. Primarily this is so that
you can use the undump program to turn your core dump into an
executable binary after having initialized all your variables at
the beginning of the program. When the new binary is executed it
will begin by executing a "goto LABEL" (with all the restrictions
that goto suffers). Think of it as a goto with an intervening
core dump and reincarnation. If LABEL is omitted, restarts the
program from the top. WARNING: any files opened at the time of
the dump will NOT be open any more when the program is
reincarnated, with possible resulting confusion on the part of
perl. See also -u.
Example:
#!/usr/bin/perl
require 'getopt.pl';
require 'stat.pl';
%days = (
'Sun',1,
'Mon',2,
'Tue',3,
'Wed',4,
'Thu',5,
'Fri',6,
'Sat',7);
dump QUICKSTART if $ARGV[0] eq '-d';
QUICKSTART:
do Getopt('f');
each(ASSOC_ARRAY)
each ASSOC_ARRAY
Returns a 2 element array consisting of the key and value for the
next value of an associative array, so that you can iterate over
it. Entries are returned in an apparently random order. When
the array is entirely read, a null array is returned (which when
assigned produces a FALSE (0) value). The next call to each()
after that will start iterating again. The iterator can be reset
only by reading all the elements from the array. You must not
modify the array while iterating over it. There is a single
iterator for each associative array, shared by all each(), keys()
and values() function calls in the program. The following prints
out your environment like the printenv program, only in a
different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also keys() and values().
eof(FILEHANDLE)
eof()
eof Returns 1 if the next read on FILEHANDLE will return end of file,
or if FILEHANDLE is not open. FILEHANDLE may be an expression
whose value gives the real filehandle name. (Note that this
function actually reads a character and then ungetc's it, so it
is not very useful in an interactive context.) An eof without an
argument returns the eof status for the last file read. Empty
parentheses () may be used to indicate the pseudo file formed of
the files listed on the command line, i.e. eof() is reasonable to
use inside a while (<>) loop to detect the end of only the last
file. Use eof(ARGV) or eof without the parentheses to test EACH
file in a while (<>) loop. Examples:
# insert dashes just before last line of last file
while (<>) {
if (eof()) {
print "--------------\n";
}
print;
}
# reset line numbering on each input file
while (<>) {
print "$.\t$_";
if (eof) { # Not eof().
close(ARGV);
}
}
eval(EXPR)
eval EXPR
eval BLOCK
EXPR is parsed and executed as if it were a little perl program.
It is executed in the context of the current perl program, so
that any variable settings, subroutine or format definitions
remain afterwards. The value returned is the value of the last
expression evaluated, just as with subroutines. If there is a
syntax error or runtime error, or a die statement is executed, an
undefined value is returned by eval, and $@ is set to the error
message. If there was no error, $@ is guaranteed to be a null
string. If EXPR is omitted, evaluates $_. The final semicolon,
if any, may be omitted from the expression.
Note that, since eval traps otherwise-fatal errors, it is useful
for determining whether a particular feature (such as dbmopen or
symlink) is implemented. It is also Perl's exception trapping
mechanism, where the die operator is used to raise exceptions.
If the code to be executed doesn't vary, you may use the eval-
BLOCK form to trap run-time errors without incurring the penalty
of recompiling each time. The error, if any, is still returned
in $@. Evaluating a single-quoted string (as EXPR) has the same
effect, except that the eval-EXPR form reports syntax errors at
run time via $@, whereas the eval-BLOCK form reports syntax
errors at compile time. The eval-EXPR form is optimized to eval-
BLOCK the first time it succeeds. (Since the replacement side of
a substitution is considered a single-quoted string when you use
the e modifier, the same optimization occurs there.) Examples:
# make divide-by-zero non-fatal
eval { $answer = $a / $b; }; warn $@ if $@;
# optimized to same thing after first use
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = };
# a run-time error
eval '$answer ='; # sets $@
exec(LIST)
exec LIST
If there is more than one argument in LIST, or if LIST is an
array with more than one value, calls execvp() with the arguments
in LIST. If there is only one scalar argument, the argument is
checked for shell metacharacters. If there are any, the entire
argument is passed to ``/bin/sh -c'' for parsing. If there are
none, the argument is split into words and passed directly to
execvp(), which is more efficient. Note: exec (and system) do
not flush your output buffer, so you may need to set $| to avoid
lost output. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argument, but want
to lie to the program you are executing about its own name, you
can specify the program you actually want to run by assigning
that to a variable and putting the name of the variable in front
of the LIST without a comma. (This always forces interpretation
of the LIST as a multi-valued list, even if there is only a
single scalar in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
exit(EXPR)
exit EXPR
Evaluates EXPR and exits immediately with that value. Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also die. If EXPR is omitted, exits with 0 status.
exp(EXPR)
exp EXPR
Returns e to the power of EXPR. If EXPR is omitted, gives
exp($_).
fcntl(FILEHANDLE,FUNCTION,SCALAR)
Implements the fcntl(2) function. You'll probably have to say
require "fcntl.ph"; # probably
/usr/local/lib/perl/fcntl.ph
first to get the correct function definitions. If fcntl.ph
doesn't exist or doesn't have the correct definitions you'll have
to roll your own, based on your C header files such as
<sys/fcntl.h>. (There is a perl script called h2ph that comes
with the perl kit which may help you in this.) Argument
processing and value return works just like ioctl below. Note
that fcntl will produce a fatal error if used on a machine that
doesn't implement fcntl(2).
fileno(FILEHANDLE)
fileno FILEHANDLE
Returns the file descriptor for a filehandle. Useful for
constructing bitmaps for select(). If FILEHANDLE is an
expression, the value is taken as the name of the filehandle.
flock(FILEHANDLE,OPERATION)
Calls flock(2) on FILEHANDLE. See manual page for flock(2) for
definition of OPERATION. Returns true for success, false on
failure. Will produce a fatal error if used on a machine that
doesn't implement flock(2). Here's a mailbox appender for BSD
systems.
$LOCK_SH = 1;
$LOCK_EX = 2;
$LOCK_NB = 4;
$LOCK_UN = 8;
sub lock {
flock(MBOX,$LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
sub unlock {
flock(MBOX,$LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
|| die "Can't open mailbox: $!";
do lock();
print MBOX $msg,"\n\n";
do unlock();
fork Does a fork() call. Returns the child pid to the parent process
and 0 to the child process. Note: unflushed buffers remain
unflushed in both processes, which means you may need to set $|
to avoid duplicate output.
getc(FILEHANDLE)
getc FILEHANDLE
getc Returns the next character from the input file attached to
FILEHANDLE, or a null string at EOF. If FILEHANDLE is omitted,
reads from STDIN.
getlogin
Returns the current login from /etc/utmp, if any. If null, use
getpwuid.
$login = getlogin || (getpwuid($<))[0] || "Somebody";
getpeername(SOCKET)
Returns the packed sockaddr address of other end of the SOCKET
connection.
# An internet sockaddr
$sockaddr = 'S n a4 x8';
$hersockaddr = getpeername(S);
($family, $port, $heraddr) =
unpack($sockaddr,$hersockaddr);
getpgrp(PID)
getpgrp PID
Returns the current process group for the specified PID, 0 for
the current process. Will produce a fatal error if used on a
machine that doesn't implement getpgrp(2). If EXPR is omitted,
returns process group of current process.
getppid Returns the process id of the parent process.
getpriority(WHICH,WHO)
Returns the current priority for a process, a process group, or a
user. (See getpriority(2).) Will produce a fatal error if used
on a machine that doesn't implement getpriority(2).
getpwnam(NAME)
getgrnam(NAME)
gethostbyname(NAME)
getnetbyname(NAME)
getprotobyname(NAME)
getpwuid(UID)
getgrgid(GID)
getservbyname(NAME,PROTO)
gethostbyaddr(ADDR,ADDRTYPE)
getnetbyaddr(ADDR,ADDRTYPE)
getprotobynumber(NUMBER)
getservbyport(PORT,PROTO)
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent(STAYOPEN)
setnetent(STAYOPEN)
setprotoent(STAYOPEN)
setservent(STAYOPEN)
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
These routines perform the same functions as their counterparts
in the system library. Within an array context, the return
values from the various get routines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell) = getpw...
($name,$passwd,$gid,$members) = getgr...
($name,$aliases,$addrtype,$length,@addrs) = gethost...
($name,$aliases,$addrtype,$net) = getnet...
($name,$aliases,$proto) = getproto...
($name,$aliases,$port,$proto) = getserv...
(If the entry doesn't exist you get a null list.)
Within a scalar context, you get the name, unless the function
was a lookup by name, in which case you get the other thing,
whatever it is. (If the entry doesn't exist you get the
undefined value.) For example:
$uid = getpwnam
$name = getpwuid
$name = getpwent
$gid = getgrnam
$name = getgrgid
$name = getgrent
etc.
The $members value returned by getgr... is a space separated list
of the login names of the members of the group.
For the gethost... functions, if the h_errno variable is
supported in C, it will be returned to you via $? if the function
call fails. The @addrs value returned by a successful call is a
list of the raw addresses returned by the corresponding system
library call. In the Internet domain, each address is four bytes
long and you can unpack it by saying something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
getsockname(SOCKET)
Returns the packed sockaddr address of this end of the SOCKET
connection.
# An internet sockaddr
$sockaddr = 'S n a4 x8';
$mysockaddr = getsockname(S);
($family, $port, $myaddr) =
unpack($sockaddr,$mysockaddr);
getsockopt(SOCKET,LEVEL,OPTNAME)
Returns the socket option requested, or undefined if there is an
error.
gmtime(EXPR)
gmtime EXPR
Converts a time as returned by the time function to a 9-element
array with the time analyzed for the Greenwich timezone.
Typically used as follows:
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
gmtime(time);
All array elements are numeric, and come straight out of a struct
tm. In particular this means that $mon has the range 0..11 and
$wday has the range 0..6. If EXPR is omitted, does gmtime(time).
goto LABEL
Finds the statement labeled with LABEL and resumes execution
there. Currently you may only go to statements in the main body
of the program that are not nested inside a do {} construct.
This statement is not implemented very efficiently, and is here
only to make the sed-to-perl translator easier. I may change its
semantics at any time, consistent with support for translated sed
scripts. Use it at your own risk. Better yet, don't use it at
all.
grep(EXPR,LIST)
Evaluates EXPR for each element of LIST (locally setting $_ to
each element) and returns the array value consisting of those
elements for which the expression evaluated to true. In a scalar
context, returns the number of times the expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
Note that, since $_ is a reference into the array value, it can
be used to modify the elements of the array. While this is
useful and supported, it can cause bizarre results if the LIST is
not a named array.
hex(EXPR)
hex EXPR
Returns the decimal value of EXPR interpreted as an hex string.
(To interpret strings that might start with 0 or 0x see oct().)
If EXPR is omitted, uses $_.
index(STR,SUBSTR,POSITION)
index(STR,SUBSTR)
Returns the position of the first occurrence of SUBSTR in STR at
or after POSITION. If POSITION is omitted, starts searching from
the beginning of the string. The return value is based at 0, or
whatever you've set the $[ variable to. If the substring is not
found, returns one less than the base, ordinarily -1.
int(EXPR)
int EXPR
Returns the integer portion of EXPR. If EXPR is omitted, uses
$_.
ioctl(FILEHANDLE,FUNCTION,SCALAR)
Implements the ioctl(2) function. You'll probably have to say
require "ioctl.ph"; # probably
/usr/local/lib/perl/ioctl.ph
first to get the correct function definitions. If ioctl.ph
doesn't exist or doesn't have the correct definitions you'll have
to roll your own, based on your C header files such as
<sys/ioctl.h>. (There is a perl script called h2ph that comes
with the perl kit which may help you in this.) SCALAR will be
read and/or written depending on the FUNCTION----a pointer to the
string value of SCALAR will be passed as the third argument of
the actual ioctl call. (If SCALAR has no string value but does
have a numeric value, that value will be passed rather than a
pointer to the string value. To guarantee this to be true, add a
0 to the scalar before using it.) The pack() and unpack()
functions are useful for manipulating the values of structures
used by ioctl(). The following example sets the erase character
to DEL.
require 'ioctl.ph';
$sgttyb_t = "ccccs"; # 4 chars and a short
if (ioctl(STDIN,$TIOCGETP,$sgttyb)) {
@ary = unpack($sgttyb_t,$sgttyb);
$ary[2] = 127;
$sgttyb = pack($sgttyb_t,@ary);
ioctl(STDIN,$TIOCSETP,$sgttyb)
|| die "Can't ioctl: $!";
}
The return value of ioctl (and fcntl) is as follows:
if OS returns: perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus perl returns true on success and false on failure, yet you
can still easily determine the actual value returned by the
operating system:
($retval = ioctl(...)) || ($retval = -1);
printf "System returned %d\n", $retval;
join(EXPR,LIST)
join(EXPR,ARRAY)
Joins the separate strings of LIST or ARRAY into a single string
with fields separated by the value of EXPR, and returns the
string. Example:
$_ = join(':',
$login,$passwd,$uid,$gid,$gcos,$home,$shell);
See split.
keys(ASSOC_ARRAY)
keys ASSOC_ARRAY
Returns a normal array consisting of all the keys of the named
associative array. The keys are returned in an apparently random
order, but it is the same order as either the values() or each()
function produces (given that the associative array has not been
modified). Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while ($#keys >= 0) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
kill(LIST)
kill LIST
Sends a signal to a list of processes. The first element of the
list must be the signal to send. Returns the number of processes
successfully signaled.
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
If the signal is negative, kills process groups instead of
processes. (On System V, a negative process number will also
kill process groups, but that's not portable.) You may use a
signal name in quotes.
last LABEL
last The last command is like the break statement in C (as used in
loops); it immediately exits the loop in question. If the LABEL
is omitted, the command refers to the innermost enclosing loop.
The continue block, if any, is not executed:
line: while (<STDIN>) {
last line if /^$/; # exit when done with header
...
}
length(EXPR)
length EXPR
Returns the length in characters of the value of EXPR. If EXPR
is omitted, returns length of $_.
link(OLDFILE,NEWFILE)
Creates a new filename linked to the old filename. Returns 1 for
success, 0 otherwise.
listen(SOCKET,QUEUESIZE)
Does the same thing that the listen system call does. Returns
true if it succeeded, false otherwise. See example in section on
Interprocess Communication.
local(LIST)
Declares the listed variables to be local to the enclosing block,
subroutine, eval or ``do''. All the listed elements must be
legal lvalues. This operator works by saving the current values
of those variables in LIST on a hidden stack and restoring them
upon exiting the block, subroutine or eval. This means that
called subroutines can also reference the local variable, but not
the global one. The LIST may be assigned to if desired, which
allows you to initialize your local variables. (If no
initializer is given for a particular variable, it is created
with an undefined value.) Commonly this is used to name the
parameters to a subroutine. Examples:
sub RANGEVAL {
local($min, $max, $thunk) = @_;
local($result) = '';
local($i);
# Presumably $thunk makes reference to $i
for ($i = $min; $i < $max; $i++) {
$result .= eval $thunk;
}
$result;
}
if ($sw eq '-v') {
# init local array with global array
local(@ARGV) = @ARGV;
unshift(@ARGV,'echo');
system @ARGV;
}
# @ARGV restored
# temporarily add to digits associative array
if ($base12) {
# (NOTE: not claiming this is efficient!)
local(%digits) = (%digits,'t',10,'e',11);
do parse_num();
}
Note that local() is a run-time command, and so gets executed
every time through a loop, using up more stack storage each time
until it's all released at once when the loop is exited.
localtime(EXPR)
localtime EXPR
Converts a time as returned by the time function to a 9-element
array with the time analyzed for the local timezone. Typically
used as follows:
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
All array elements are numeric, and come straight out of a struct
tm. In particular this means that $mon has the range 0..11 and
$wday has the range 0..6. If EXPR is omitted, does
localtime(time).
log(EXPR)
log EXPR
Returns logarithm (base e) of EXPR. If EXPR is omitted, returns
log of $_.
lstat(FILEHANDLE)
lstat FILEHANDLE
lstat(EXPR)
lstat SCALARVARIABLE
Does the same thing as the stat() function, but stats a symbolic
link instead of the file the symbolic link points to. If
symbolic links are unimplemented on your system, a normal stat is
done.
m/PATTERN/gio
/PATTERN/gio
Searches a string for a pattern match, and returns true (1) or
false (''). If no string is specified via the =~ or !~ operator,
the $_ string is searched. (The string specified with =~ need
not be an lvalue----it may be the result of an expression
evaluation, but remember the =~ binds rather tightly.) See also
the section on regular expressions.
If / is the delimiter then the initial `m' is optional. With the
`m' you can use any pair of non-alphanumeric characters as
delimiters. This is particularly useful for matching Unix path
names that contain `/'. If the final delimiter is followed by
the optional letter `i', the matching is done in a case-
insensitive manner. PATTERN may contain references to scalar
variables, which will be interpolated (and the pattern
recompiled) every time the pattern search is evaluated. (Note
that $) and $| may not be interpolated because they look like
end-of-string tests.) If you want such a pattern to be compiled
only once, add an ``o'' after the trailing delimiter. This
avoids expensive run-time recompilations, and is useful when the
value you are interpolating won't change over the life of the
script. If the PATTERN evaluates to a null string, the most
recent successful regular expression is used instead.
If used in a context that requires an array value, a pattern
match returns an array consisting of the subexpressions matched
by the parentheses in the pattern, i.e. ($1, $2, $3...). It does
NOT actually set $1, $2, etc. in this case, nor does it set $+,
$`, $& or $'. If the match fails, a null array is returned. If
the match succeeds, but there were no parentheses, an array value
of (1) is returned.
Examples:
open(tty, '/dev/tty');
<tty> =~ /^y/i && do foo(); # do foo if desired
if (/Version: *([0-9.]*)/) { $version = $1; }
next if m#^/usr/spool/uucp#;
# poor man's grep
$arg = shift;
while (<>) {
print if /$arg/o; # compile only once
}
if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
This last example splits $foo into the first two words and the
remainder of the line, and assigns those three fields to $F1, $F2
and $Etc. The conditional is true if any variables were
assigned, i.e. if the pattern matched.
The ``g'' modifier specifies global pattern matching----that is,
matching as many times as possible within the string. How it
behaves depends on the context. In an array context, it returns
a list of all the substrings matched by all the parentheses in
the regular expression. If there are no parentheses, it returns
a list of all the matched strings, as if there were parentheses
around the whole pattern. In a scalar context, it iterates
through the string, returning TRUE each time it matches, and
FALSE when it eventually runs out of matches. (In other words,
it remembers where it left off last time and restarts the search
at that point.) It presumes that you have not modified the
string since the last match. Modifying the string between
matches may result in undefined behavior. (You can actually get
away with in-place modifications via substr() that do not change
the length of the entire string. In general, however, you should
be using s///g for such modifications.) Examples:
# array context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
$/ = ""; $* = 1;
while ($paragraph = <>) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
print "$sentences\n";
mkdir(FILENAME,MODE)
Creates the directory specified by FILENAME, with permissions
specified by MODE (as modified by umask). If it succeeds it
returns 1, otherwise it returns 0 and sets $! (errno).
msgctl(ID,CMD,ARG)
Calls the System V IPC function msgctl. If CMD is &IPC_STAT,
then ARG must be a variable which will hold the returned msqid_ds
structure. Returns like ioctl: the undefined value for error, "0
but true" for zero, or the actual return value otherwise.
msgget(KEY,FLAGS)
Calls the System V IPC function msgget. Returns the message
queue id, or the undefined value if there is an error.
msgsnd(ID,MSG,FLAGS)
Calls the System V IPC function msgsnd to send the message MSG to
the message queue ID. MSG must begin with the long integer
message type, which may be created with pack("L", $type).
Returns true if successful, or false if there is an error.
msgrcv(ID,VAR,SIZE,TYPE,FLAGS)
Calls the System V IPC function msgrcv to receive a message from
message queue ID into variable VAR with a maximum message size of
SIZE. Note that if a message is received, the message type will
be the first thing in VAR, and the maximum length of VAR is SIZE
plus the size of the message type. Returns true if successful,
or false if there is an error.
next LABEL
next The next command is like the continue statement in C; it starts
the next iteration of the loop:
line: while (<STDIN>) {
next line if /^#/; # discard comments
...
}
Note that if there were a continue block on the above, it would
get executed even on discarded lines. If the LABEL is omitted,
the command refers to the innermost enclosing loop.
oct(EXPR)
oct EXPR
Returns the decimal value of EXPR interpreted as an octal string.
(If EXPR happens to start off with 0x, interprets it as a hex
string instead.) The following will handle decimal, octal and
hex in the standard notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_.
open(FILEHANDLE,EXPR)
open(FILEHANDLE)
open FILEHANDLE
Opens the file whose filename is given by EXPR, and associates it
with FILEHANDLE. If FILEHANDLE is an expression, its value is
used as the name of the real filehandle wanted. If EXPR is
omitted, the scalar variable of the same name as the FILEHANDLE
contains the filename. If the filename begins with ``<'' or
nothing, the file is opened for input. If the filename begins
with ``>'', the file is opened for output. If the filename
begins with ``>>'', the file is opened for appending. (You can
put a '+' in front of the '>' or '<' to indicate that you want
both read and write access to the file.) If the filename begins
with ``|'', the filename is interpreted as a command to which
output is to be piped, and if the filename ends with a ``|'', the
filename is interpreted as command which pipes input to us. (You
may not have a command that pipes both in and out.) Opening '-'
opens STDIN and opening '>-' opens STDOUT. Open returns non-zero
upon success, the undefined value otherwise. If the open
involved a pipe, the return value happens to be the pid of the
subprocess. Examples:
$article = 100;
open article || die "Can't find article $article: $!\n";
while (<article>) {...
open(LOG, '>>/usr/spool/news/twitlog');
# (log is reserved)
open(article, "caesar <$article |");
# decrypt article
open(extract, "|sort >/tmp/Tmp$$");
# $$ is our process#
# process argument list of files along with any includes
foreach $file (@ARGV) {
do process($file, 'fh00'); # no pun intended
}
sub process {
local($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename:
$!\n";
return;
}
while (<$input>) { # note the use of indirection
if (/^#include "(.*)"/) {
do process($1, $input);
next;
}
... # whatever
}
}
You may also, in the Bourne shell tradition, specify an EXPR
beginning with ``>&'', in which case the rest of the string is
interpreted as the name of a filehandle (or file descriptor, if
numeric) which is to be duped and opened. You may use & after >,
>>, <, +>, +>> and +<. The mode you specify should match the
mode of the original filehandle. Here is a script that saves,
redirects, and restores STDOUT and STDERR:
#!/usr/bin/perl
open(SAVEOUT, ">&STDOUT");
open(SAVEERR, ">&STDERR");
open(STDOUT, ">foo.out") || die "Can't redirect stdout";
open(STDERR, ">&STDOUT") || die "Can't dup stdout";
select(STDERR); $| = 1; # make unbuffered
select(STDOUT); $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
close(STDOUT);
close(STDERR);
open(STDOUT, ">&SAVEOUT");
open(STDERR, ">&SAVEERR");
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you open a pipe on the command ``-'', i.e. either ``|-'' or
``-|'', then there is an implicit fork done, and the return value
of open is the pid of the child within the parent process, and 0
within the child process. (Use defined($pid) to determine if the
open was successful.) The filehandle behaves normally for the
parent, but i/o to that filehandle is piped from/to the
STDOUT/STDIN of the child process. In the child process the
filehandle isn't opened----i/o happens from/to the new STDOUT or
STDIN. Typically this is used like the normal piped open when
you want to exercise more control over just how the pipe command
gets executed, such as when you are running setuid, and don't
want to have to scan shell commands for metacharacters. The
following pairs are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, "cat -n '$file'|");
open(FOO, "-|") || exec 'cat', '-n', $file;
Explicitly closing any piped filehandle causes the parent process
to wait for the child to finish, and returns the status value in
$?. Note: on any operation which may do a fork, unflushed
buffers remain unflushed in both processes, which means you may
need to set $| to avoid duplicate output.
The filename that is passed to open will have leading and
trailing whitespace deleted. In order to open a file with
arbitrary weird characters in it, it's necessary to protect any
leading and trailing whitespace thusly:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
opendir(DIRHANDLE,EXPR)
Opens a directory named EXPR for processing by readdir(),
telldir(), seekdir(), rewinddir() and closedir(). Returns true
if successful. DIRHANDLEs have their own namespace separate from
FILEHANDLEs.
ord(EXPR)
ord EXPR
Returns the numeric ascii value of the first character of EXPR.
If EXPR is omitted, uses $_.
pack(TEMPLATE,LIST)
Takes an array or list of values and packs it into a binary
structure, returning the string containing the structure. The
TEMPLATE is a sequence of characters that give the order and type
of values, as follows:
A An ascii string, will be space padded.
a An ascii string, will be null padded.
c A signed char value.
C An unsigned char value.
s A signed short value.
S An unsigned short value.
i A signed integer value.
I An unsigned integer value.
l A signed long value.
L An unsigned long value.
n A short in ``network'' order.
N A long in ``network'' order.
f A single-precision float in the native format.
d A double-precision float in the native format.
p A pointer to a string.
v A short in ``VAX'' (little-endian) order.
V A long in ``VAX'' (little-endian) order.
x A null byte.
X Back up a byte.
@ Null fill to absolute position.
u A uuencoded string.
b A bit string (ascending bit order, like vec()).
B A bit string (descending bit order).
h A hex string (low nybble first).
H A hex string (high nybble first).
Each letter may optionally be followed by a number which gives a
repeat count. With all types except "a", "A", "b", "B", "h" and
"H", the pack function will gobble up that many values from the
LIST. A * for the repeat count means to use however many items
are left. The "a" and "A" types gobble just one value, but pack
it as a string of length count, padding with nulls or spaces as
necessary. (When unpacking, "A" strips trailing spaces and
nulls, but "a" does not.) Likewise, the "b" and "B" fields pack
a string that many bits long. The "h" and "H" fields pack a
string that many nybbles long. Real numbers (floats and doubles)
are in the native machine format only; due to the multiplicity of
floating formats around, and the lack of a standard ``network''
representation, no facility for interchange has been made. This
means that packed floating point data written on one machine may
not be readable on another - even if both use IEEE floating point
arithmetic (as the endian-ness of the memory representation is
not part of the IEEE spec). Note that perl uses doubles
internally for all numeric calculation, and converting from
double -> float -> double will lose precision (i.e. unpack("f",
pack("f", $foo)) will not in general equal $foo).
Examples:
$foo = pack("cccc",65,66,67,68);
# foo eq "ABCD"
$foo = pack("c4",65,66,67,68);
# same thing
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift,
-32)));
}
The same template may generally also be used in the unpack
function.
pipe(READHANDLE,WRITEHANDLE)
Opens a pair of connected pipes like the corresponding system
call. Note that if you set up a loop of piped processes,
deadlock can occur unless you are very careful. In addition,
note that perl's pipes use stdio buffering, so you may need to
set $| to flush your WRITEHANDLE after each command, depending on
the application. [Requires version 3.0 patchlevel 9.]
pop(ARRAY)
pop ARRAY
Pops and returns the last value of the array, shortening the
array by 1. Has the same effect as
$tmp = $ARRAY[$#ARRAY--];
If there are no elements in the array, returns the undefined
value.
print(FILEHANDLE LIST)
print(LIST)
print FILEHANDLE LIST
print LIST
print Prints a string or a comma-separated list of strings. Returns
non-zero if successful. FILEHANDLE may be a scalar variable
name, in which case the variable contains the name of the
filehandle, thus introducing one level of indirection. (NOTE: If
FILEHANDLE is a variable and the next token is a term, it may be
misinterpreted as an operator unless you interpose a + or put
parens around the arguments.) If FILEHANDLE is omitted, prints
by default to standard output (or to the last selected output
channel----see select()). If LIST is also omitted, prints $_ to
STDOUT. To set the default output channel to something other
than STDOUT use the select operation. Note that, because print
takes a LIST, anything in the LIST is evaluated in an array
context, and any subroutine that you call will have one or more
of its expressions evaluated in an array context. Also be
careful not to follow the print keyword with a left parenthesis
unless you want the corresponding right parenthesis to terminate
the arguments to the print----interpose a + or put parens around
all the arguments.
printf(FILEHANDLE LIST)
printf(LIST)
printf FILEHANDLE LIST
printf LIST
Equivalent to a ``print FILEHANDLE sprintf(LIST)''.
push(ARRAY,LIST)
Treats ARRAY (@ is optional) as a stack, and pushes the values of
LIST onto the end of ARRAY. The length of ARRAY increases by the
length of LIST. Has the same effect as
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient.
q/STRING/
qq/STRING/
qx/STRING/
These are not really functions, but simply syntactic sugar to let
you avoid putting too many backslashes into quoted strings. The
q operator is a generalized single quote, and the qq operator a
generalized double quote. The qx operator is a generalized
backquote. Any non-alphanumeric delimiter can be used in place
of /, including newline. If the delimiter is an opening bracket
or parenthesis, the final delimiter will be the corresponding
closing bracket or parenthesis. (Embedded occurrences of the
closing bracket need to be backslashed as usual.) Examples:
$foo = q!I said, "You said, 'She said it.'"!;
$bar = q('This is it.');
$today = qx{ date };
$_ .= qq
*** The previous line contains the naughty word "$&".\n
if /(ibm|apple|awk)/; # :-)
rand(EXPR)
rand EXPR
rand Returns a random fractional number between 0 and the value of
EXPR. (EXPR should be positive.) If EXPR is omitted, returns a
value between 0 and 1. See also srand().
read(FILEHANDLE,SCALAR,LENGTH,OFFSET)
read(FILEHANDLE,SCALAR,LENGTH)
Attempts to read LENGTH bytes of data into variable SCALAR from
the specified FILEHANDLE. Returns the number of bytes actually
read, or undef if there was an error. SCALAR will be grown or
shrunk to the length actually read. An OFFSET may be specified
to place the read data at some other place than the beginning of
the string. This call is actually implemented in terms of
stdio's fread call. To get a true read system call, see sysread.
readdir(DIRHANDLE)
readdir DIRHANDLE
Returns the next directory entry for a directory opened by
opendir(). If used in an array context, returns all the rest of
the entries in the directory. If there are no more entries,
returns an undefined value in a scalar context or a null list in
an array context.
readlink(EXPR)
readlink EXPR
Returns the value of a symbolic link, if symbolic links are
implemented. If not, gives a fatal error. If there is some
system error, returns the undefined value and sets $! (errno).
If EXPR is omitted, uses $_.
recv(SOCKET,SCALAR,LEN,FLAGS)
Receives a message on a socket. Attempts to receive LENGTH bytes
of data into variable SCALAR from the specified SOCKET
filehandle. Returns the address of the sender, or the undefined
value if there's an error. SCALAR will be grown or shrunk to the
length actually read. Takes the same flags as the system call of
the same name.
redo LABEL
redo The redo command restarts the loop block without evaluating the
conditional again. The continue block, if any, is not executed.
If the LABEL is omitted, the command refers to the innermost
enclosing loop. This command is normally used by programs that
want to lie to themselves about what was just input:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
line: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front{|;
redo line;
}
}
}
print;
}
rename(OLDNAME,NEWNAME)
Changes the name of a file. Returns 1 for success, 0 otherwise.
Will not work across filesystem boundaries.
require(EXPR)
require EXPR
require Includes the library file specified by EXPR, or by $_ if EXPR is
not supplied. Has semantics similar to the following subroutine:
sub require {
local($filename) = @_;
return 1 if $INC{$filename};
local($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
die $@ if $@;
die "$filename did not return true value" unless
$result;
$INC{$filename} = $realfilename;
$result;
}
Note that the file will not be included twice under the same
specified name. The file must return true as the last statement
to indicate successful execution of any initialization code, so
it's customary to end such a file with ``1;'' unless you're sure
it'll return true otherwise.
reset(EXPR)
reset EXPR
reset Generally used in a continue block at the end of a loop to clear
variables and reset ?? searches so that they work again. The
expression is interpreted as a list of single characters (hyphens
allowed for ranges). All variables and arrays beginning with one
of those letters are reset to their pristine state. If the
expression is omitted, one-match searches (?pattern?) are reset
to match again. Only resets variables or searches in the current
package. Always returns 1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?? searches
Note: resetting ``A-Z'' is not recommended since you'll wipe out
your ARGV and ENV arrays.
The use of reset on dbm associative arrays does not change the
dbm file. (It does, however, flush any entries cached by perl,
which may be useful if you are sharing the dbm file. Then again,
maybe not.)
return LIST
Returns from a subroutine with the value specified. (Note that a
subroutine can automatically return the value of the last
expression evaluated. That's the preferred method----use of an
explicit return is a bit slower.)
reverse(LIST)
reverse LIST
In an array context, returns an array value consisting of the
elements of LIST in the opposite order. In a scalar context,
returns a string value consisting of the bytes of the first
element of LIST in the opposite order.
rewinddir(DIRHANDLE)
rewinddir DIRHANDLE
Sets the current position to the beginning of the directory for
the readdir() routine on DIRHANDLE.
rindex(STR,SUBSTR,POSITION)
rindex(STR,SUBSTR)
Works just like index except that it returns the position of the
LAST occurrence of SUBSTR in STR. If POSITION is specified,
returns the last occurrence at or before that position.
rmdir(FILENAME)
rmdir FILENAME
Deletes the directory specified by FILENAME if it is empty. If
it succeeds it returns 1, otherwise it returns 0 and sets $!
(errno). If FILENAME is omitted, uses $_.
s/PATTERN/REPLACEMENT/gieo
Searches a string for a pattern, and if found, replaces that
pattern with the replacement text and returns the number of
substitutions made. Otherwise it returns false (0). The ``g''
is optional, and if present, indicates that all occurrences of
the pattern are to be replaced. The ``i'' is also optional, and
if present, indicates that matching is to be done in a case-
insensitive manner. The ``e'' is likewise optional, and if
present, indicates that the replacement string is to be evaluated
as an expression rather than just as a double-quoted string. Any
non-alphanumeric delimiter may replace the slashes; if single
quotes are used, no interpretation is done on the replacement
string (the e modifier overrides this, however); if backquotes
are used, the replacement string is a command to execute whose
output will be used as the actual replacement text. If the
PATTERN is delimited by bracketing quotes, the REPLACEMENT has
its own pair of quotes, which may or may not be bracketing
quotes, e.g. s(foo)(bar) or s<foo>/bar/. If no string is
specified via the =~ or !~ operator, the $_ string is searched
and modified. (The string specified with =~ must be a scalar
variable, an array element, or an assignment to one of those,
i.e. an lvalue.) If the pattern contains a $ that looks like a
variable rather than an end-of-string test, the variable will be
interpolated into the pattern at run-time. If you only want the
pattern compiled once the first time the variable is
interpolated, add an ``o'' at the end. If the PATTERN evaluates
to a null string, the most recent successful regular expression
is used instead. See also the section on regular expressions.
Examples:
s/\bgreen\b/mauve/g; # don't change wintergreen
$path =~ s|/usr/bin|/usr/local/bin|;
s/Login: $foo/Login: $bar/; # run-time pattern
($foo = $bar) =~ s/bar/foo/;
$_ = 'abc123xyz';
s/\d+/$&*2/e; # yields `abc246xyz'
s/\d+/sprintf("%5d",$&)/e; # yields `abc 246xyz'
s/\w/$& x 2/eg; # yields `aabbcc 224466xxyyzz'
s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
(Note the use of $ instead of \ in the last example. See section
on regular expressions.)
scalar(EXPR)
Forces EXPR to be interpreted in a scalar context and returns the
value of EXPR.
seek(FILEHANDLE,POSITION,WHENCE)
Randomly positions the file pointer for FILEHANDLE, just like the
fseek() call of stdio. FILEHANDLE may be an expression whose
value gives the name of the filehandle. Returns 1 upon success,
0 otherwise.
seekdir(DIRHANDLE,POS)
Sets the current position for the readdir() routine on DIRHANDLE.
POS must be a value returned by telldir(). Has the same caveats
about possible directory compaction as the corresponding system
library routine.
select(FILEHANDLE)
select Returns the currently selected filehandle. Sets the current
default filehandle for output, if FILEHANDLE is supplied. This
has two effects: first, a write or a print without a filehandle
will default to this FILEHANDLE. Second, references to variables
related to output will refer to this output channel. For
example, if you have to set the top of form format for more than
one output channel, you might do the following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives the name of the
actual filehandle. Thus:
$oldfh = select(STDERR); $| = 1; select($oldfh);
select(RBITS,WBITS,EBITS,TIMEOUT)
This calls the select system call with the bitmasks specified,
which can be constructed using fileno() and vec(), along these
lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles you might wish to write
a subroutine:
sub fhbits {
local(@fhlist) = split(' ',$_[0]);
local($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = &fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready:
$nfound = select($rout=$rin, $wout=$win,
$eout=$ein, undef);
Any of the bitmasks can also be undef. The timeout, if
specified, is in seconds, which may be fractional. NOTE: not all
implementations are capable of returning the $timeleft. If not,
they always return $timeleft equal to the supplied $timeout.
semctl(ID,SEMNUM,CMD,ARG)
Calls the System V IPC function semctl. If CMD is &IPC_STAT or
&GETALL, then ARG must be a variable which will hold the returned
semid_ds structure or semaphore value array. Returns like ioctl:
the undefined value for error, "0 but true" for zero, or the
actual return value otherwise.
semget(KEY,NSEMS,SIZE,FLAGS)
Calls the System V IPC function semget. Returns the semaphore
id, or the undefined value if there is an error.
semop(KEY,OPSTRING)
Calls the System V IPC function semop to perform semaphore
operations such as signaling and waiting. OPSTRING must be a
packed array of semop structures. Each semop structure can be
generated with 'pack("sss", $semnum, $semop, $semflag)'. The
number of semaphore operations is implied by the length of
OPSTRING. Returns true if successful, or false if there is an
error. As an example, the following code waits on semaphore
$semnum of semaphore id $semid:
$semop = pack("sss", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid,
$semop);
To signal the semaphore, replace "-1" with "1".
send(SOCKET,MSG,FLAGS,TO)
send(SOCKET,MSG,FLAGS)
Sends a message on a socket. Takes the same flags as the system
call of the same name. On unconnected sockets you must specify a
destination to send TO. Returns the number of characters sent,
or the undefined value if there is an error.
setpgrp(PID,PGRP)
Sets the current process group for the specified PID, 0 for the
current process. Will produce a fatal error if used on a machine
that doesn't implement setpgrp(2).
setpriority(WHICH,WHO,PRIORITY)
Sets the current priority for a process, a process group, or a
user. (See setpriority(2).) Will produce a fatal error if used
on a machine that doesn't implement setpriority(2).
setsockopt(SOCKET,LEVEL,OPTNAME,OPTVAL)
Sets the socket option requested. Returns undefined if there is
an error. OPTVAL may be specified as undef if you don't want to
pass an argument.
shift(ARRAY)
shift ARRAY
shift Shifts the first value of the array off and returns it,
shortening the array by 1 and moving everything down. If there
are no elements in the array, returns the undefined value. If
ARRAY is omitted, shifts the @ARGV array in the main program, and
the @_ array in subroutines. (This is determined lexically.)
See also unshift(), push() and pop(). Shift() and unshift() do
the same thing to the left end of an array that push() and pop()
do to the right end.
shmctl(ID,CMD,ARG)
Calls the System V IPC function shmctl. If CMD is &IPC_STAT,
then ARG must be a variable which will hold the returned shmid_ds
structure. Returns like ioctl: the undefined value for error, "0
but true" for zero, or the actual return value otherwise.
shmget(KEY,SIZE,FLAGS)
Calls the System V IPC function shmget. Returns the shared
memory segment id, or the undefined value if there is an error.
shmread(ID,VAR,POS,SIZE)
shmwrite(ID,STRING,POS,SIZE)
Reads or writes the System V shared memory segment ID starting at
position POS for size SIZE by attaching to it, copying in/out,
and detaching from it. When reading, VAR must be a variable
which will hold the data read. When writing, if STRING is too
long, only SIZE bytes are used; if STRING is too short, nulls are
written to fill out SIZE bytes. Return true if successful, or
false if there is an error.
shutdown(SOCKET,HOW)
Shuts down a socket connection in the manner indicated by HOW,
which has the same interpretation as in the system call of the
same name.
sin(EXPR)
sin EXPR
Returns the sine of EXPR (expressed in radians). If EXPR is
omitted, returns sine of $_.
sleep(EXPR)
sleep EXPR
sleep Causes the script to sleep for EXPR seconds, or forever if no
EXPR. May be interrupted by sending the process a SIGALRM.
Returns the number of seconds actually slept. You probably
cannot mix alarm() and sleep() calls, since sleep() is often
implemented using alarm().
socket(SOCKET,DOMAIN,TYPE,PROTOCOL)
Opens a socket of the specified kind and attaches it to
filehandle SOCKET. DOMAIN, TYPE and PROTOCOL are specified the
same as for the system call of the same name. You may need to
run h2ph on sys/socket.h to get the proper values handy in a perl
library file. Return true if successful. See the example in the
section on Interprocess Communication.
socketpair(SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL)
Creates an unnamed pair of sockets in the specified domain, of
the specified type. DOMAIN, TYPE and PROTOCOL are specified the
same as for the system call of the same name. If unimplemented,
yields a fatal error. Return true if successful.
sort(SUBROUTINE LIST)
sort(LIST)
sort SUBROUTINE LIST
sort BLOCK LIST
sort LIST
Sorts the LIST and returns the sorted array value. Nonexistent
values of arrays are stripped out. If SUBROUTINE or BLOCK is
omitted, sorts in standard string comparison order. If
SUBROUTINE is specified, gives the name of a subroutine that
returns an integer less than, equal to, or greater than 0,
depending on how the elements of the array are to be ordered.
(The <=> and cmp operators are extremely useful in such
routines.) SUBROUTINE may be a scalar variable name, in which
case the value provides the name of the subroutine to use. In
place of a SUBROUTINE name, you can provide a BLOCK as an
anonymous, in-line sort subroutine.
In the interests of efficiency the normal calling code for
subroutines is bypassed, with the following effects: the
subroutine may not be a recursive subroutine, and the two
elements to be compared are passed into the subroutine not via @_
but as $a and $b (see example below). They are passed by
reference so don't modify $a and $b.
Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming integers
}
@sortedclass = sort byage @class;
sub reverse { $b cmp $a; }
@harry = ('dog','cat','x','Cain','Abel');
@george = ('gone','chased','yz','Punished','Axed');
print sort @harry;
# prints AbelCaincatdogx
print sort reverse @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints
AbelAxedCainPunishedcatchaseddoggonetoxyz
splice(ARRAY,OFFSET,LENGTH,LIST)
splice(ARRAY,OFFSET,LENGTH)
splice(ARRAY,OFFSET)
Removes the elements designated by OFFSET and LENGTH from an
array, and replaces them with the elements of LIST, if any.
Returns the elements removed from the array. The array grows or
shrinks as necessary. If LENGTH is omitted, removes everything
from OFFSET onward. The following equivalencies hold (assuming
$[ == 0):
push(@a,$x,$y)
splice(@a,$#a+1,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$x] = $y splice(@a,$x,1,$y);
Example, assuming array lengths are passed before arrays:
sub aeq { # compare two array values
local(@a) = splice(@_,0,shift);
local(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
split(/PATTERN/,EXPR,LIMIT)
split(/PATTERN/,EXPR)
split(/PATTERN/)
split Splits a string into an array of strings, and returns it. (If
not in an array context, returns the number of fields found and
splits into the @_ array. (In an array context, you can force
the split into @_ by using ?? as the pattern delimiters, but it
still returns the array value.)) If EXPR is omitted, splits the
$_ string. If PATTERN is also omitted, splits on whitespace
(/[ \t\n]+/). Anything matching PATTERN is taken to be a
delimiter separating the fields. (Note that the delimiter may be
longer than one character.) If LIMIT is specified, splits into
no more than that many fields (though it may split into fewer).
If LIMIT is unspecified, trailing null fields are stripped (which
potential users of pop() would do well to remember). A pattern
matching the null string (not to be confused with a null pattern
//, which is just one member of the set of patterns matching a
null string) will split the value of EXPR into separate
characters at each point it matches that way. For example:
print join(':', split(/ */, 'hi there'));
produces the output `h:i:t:h:e:r:e'.
The LIMIT parameter can be used to partially split a line
($login, $passwd, $remainder) = split(/:/, $_, 3);
(When assigning to a list, if LIMIT is omitted, perl supplies a
LIMIT one larger than the number of variables in the list, to
avoid unnecessary work. For the list above LIMIT would have been
4 by default. In time critical applications it behooves you not
to split into more fields than you really need.)
If the PATTERN contains parentheses, additional array elements
are created from each matching substring in the delimiter.
split(/([,-])/,"1-10,20");
produces the array value
(1,'-',10,',',20)
The pattern /PATTERN/ may be replaced with an expression to
specify patterns that vary at runtime. (To do runtime
compilation only once, use /$variable/o.) As a special case,
specifying a space (' ') will split on white space just as split
with no arguments does, but leading white space does NOT produce
a null first field. Thus, split(' ') can be used to emulate
awk's default behavior, whereas split(/ /) will give you as many
null initial fields as there are leading spaces.
Example:
open(passwd, '/etc/passwd');
while (<passwd>) {
($login, $passwd, $uid, $gid, $gcos, $home,
$shell)
= split(/:/);
...
}
(Note that $shell above will still have a newline on it. See
chop().) See also join.
sprintf(FORMAT,LIST)
Returns a string formatted by the usual printf conventions. The
* character is not supported.
sqrt(EXPR)
sqrt EXPR
Return the square root of EXPR. If EXPR is omitted, returns
square root of $_.
srand(EXPR)
srand EXPR
Sets the random number seed for the rand operator. If EXPR is
omitted, does srand(time).
stat(FILEHANDLE)
stat FILEHANDLE
stat(EXPR)
stat SCALARVARIABLE
Returns a 13-element array giving the statistics for a file,
either the file opened via FILEHANDLE, or named by EXPR. Returns
a null list if the stat fails. Typically used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
If stat is passed the special filehandle consisting of an
underline, no stat is done, but the current contents of the stat
structure from the last stat or filetest are returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This only works on machines for which the device number is
negative under NFS.)
study(SCALAR)
study SCALAR
study Takes extra time to study SCALAR ($_ if unspecified) in
anticipation of doing many pattern matches on the string before
it is next modified. This may or may not save time, depending on
the nature and number of patterns you are searching on, and on
the distribution of character frequencies in the string to be
searched----you probably want to compare runtimes with and
without it to see which runs faster. Those loops which scan for
many short constant strings (including the constant parts of more
complex patterns) will benefit most. You may have only one study
active at a time----if you study a different scalar the first is
``unstudied''. (The way study works is this: a linked list of
every character in the string to be searched is made, so we know,
for example, where all the `k' characters are. From each search
string, the rarest character is selected, based on some static
frequency tables constructed from some C programs and English
text. Only those places that contain this ``rarest'' character
are examined.)
For example, here is a loop which inserts index producing entries
before any line containing a certain pattern:
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
...
print;
}
In searching for /\bfoo\b/, only those locations in $_ that
contain `f' will be looked at, because `f' is rarer than `o'. In
general, this is a big win except in pathological cases. The
only question is whether it saves you more time than it took to
build the linked list in the first place.
Note that if you have to look for strings that you don't know
till runtime, you can build an entire loop as a string and eval
that to avoid recompiling all your patterns all the time.
Together with undefining $/ to input entire files as one record,
this can be very fast, often faster than specialized programs
like fgrep. The following scans a list of files (@files) for a
list of words (@words), and prints out the names of those files
that contain a match:
$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delim
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
substr(EXPR,OFFSET,LEN)
substr(EXPR,OFFSET)
Extracts a substring out of EXPR and returns it. First character
is at offset 0, or whatever you've set $[ to. If OFFSET is
negative, starts that far from the end of the string. If LEN is
omitted, returns everything to the end of the string. You can
use the substr() function as an lvalue, in which case EXPR must
be an lvalue. If you assign something shorter than LEN, the
string will shrink, and if you assign something longer than LEN,
the string will grow to accommodate it. To keep the string the
same length you may need to pad or chop your value using
sprintf().
symlink(OLDFILE,NEWFILE)
Creates a new filename symbolically linked to the old filename.
Returns 1 for success, 0 otherwise. On systems that don't
support symbolic links, produces a fatal error at run time. To
check for that, use eval:
$symlink_exists = (eval 'symlink("","");', $@ eq '');
syscall(LIST)
syscall LIST
Calls the system call specified as the first element of the list,
passing the remaining elements as arguments to the system call.
If unimplemented, produces a fatal error. The arguments are
interpreted as follows: if a given argument is numeric, the
argument is passed as an int. If not, the pointer to the string
value is passed. You are responsible to make sure a string is
pre-extended long enough to receive any result that might be
written into a string. If your integer arguments are not
literals and have never been interpreted in a numeric context,
you may need to add 0 to them to force them to look like numbers.
require 'syscall.ph'; # may need to run h2ph
syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
sysread(FILEHANDLE,SCALAR,LENGTH,OFFSET)
sysread(FILEHANDLE,SCALAR,LENGTH)
Attempts to read LENGTH bytes of data into variable SCALAR from
the specified FILEHANDLE, using the system call read(2). It
bypasses stdio, so mixing this with other kinds of reads may
cause confusion. Returns the number of bytes actually read, or
undef if there was an error. SCALAR will be grown or shrunk to
the length actually read. An OFFSET may be specified to place
the read data at some other place than the beginning of the
string.
system(LIST)
system LIST
Does exactly the same thing as ``exec LIST'' except that a fork
is done first, and the parent process waits for the child process
to complete. Note that argument processing varies depending on
the number of arguments. The return value is the exit status of
the program as returned by the wait() call. To get the actual
exit value divide by 256. See also exec.
syswrite(FILEHANDLE,SCALAR,LENGTH,OFFSET)
syswrite(FILEHANDLE,SCALAR,LENGTH)
Attempts to write LENGTH bytes of data from variable SCALAR to
the specified FILEHANDLE, using the system call write(2). It
bypasses stdio, so mixing this with prints may cause confusion.
Returns the number of bytes actually written, or undef if there
was an error. An OFFSET may be specified to place the read data
at some other place than the beginning of the string.
tell(FILEHANDLE)
tell FILEHANDLE
tell Returns the current file position for FILEHANDLE. FILEHANDLE may
be an expression whose value gives the name of the actual
filehandle. If FILEHANDLE is omitted, assumes the file last
read.
telldir(DIRHANDLE)
telldir DIRHANDLE
Returns the current position of the readdir() routines on
DIRHANDLE. Value may be given to seekdir() to access a
particular location in a directory. Has the same caveats about
possible directory compaction as the corresponding system library
routine.
time Returns the number of non-leap seconds since 00:00:00 UTC,
January 1, 1970. Suitable for feeding to gmtime() and
localtime().
times Returns a four-element array giving the user and system times, in
seconds, for this process and the children of this process.
($user,$system,$cuser,$csystem) = times;
tr/SEARCHLIST/REPLACEMENTLIST/cds
y/SEARCHLIST/REPLACEMENTLIST/cds
Translates all occurrences of the characters found in the search
list with the corresponding character in the replacement list.
It returns the number of characters replaced or deleted. If no
string is specified via the =~ or !~ operator, the $_ string is
translated. (The string specified with =~ must be a scalar
variable, an array element, or an assignment to one of those,
i.e. an lvalue.) For sed devotees, y is provided as a synonym
for tr. If the SEARCHLIST is delimited by bracketing quotes, the
REPLACEMENTLIST has its own pair of quotes, which may or may not
be bracketing quotes, e.g. tr[A-Z][a-z] or tr(+-*/)/ABCD/.
If the c modifier is specified, the SEARCHLIST character set is
complemented. If the d modifier is specified, any characters
specified by SEARCHLIST that are not found in REPLACEMENTLIST are
deleted. (Note that this is slightly more flexible than the
behavior of some tr programs, which delete anything they find in
the SEARCHLIST, period.) If the s modifier is specified,
sequences of characters that were translated to the same
character are squashed down to 1 instance of the character.
If the d modifier was used, the REPLACEMENTLIST is always
interpreted exactly as specified. Otherwise, if the
REPLACEMENTLIST is shorter than the SEARCHLIST, the final
character is replicated till it is long enough. If the
REPLACEMENTLIST is null, the SEARCHLIST is replicated. This
latter is useful for counting characters in a class, or for
squashing character sequences in a class.
Examples:
$ARGV[1] =~ y/A-Z/a-z/; # canonicalize to lower case
$cnt = tr/*/*/; # count the stars in $_
$cnt = tr/0-9//; # count the digits in $_
tr/a-zA-Z//s; # bookkeeper -> bokeper
($HOST = $host) =~ tr/a-z/A-Z/;
y/a-zA-Z/ /cs; # change non-alphas to single
space
tr/\200-\377/\0-\177/; # delete 8th bit
truncate(FILEHANDLE,LENGTH)
truncate(EXPR,LENGTH)
Truncates the file opened on FILEHANDLE, or named by EXPR, to the
specified length. Produces a fatal error if truncate isn't
implemented on your system.
umask(EXPR)
umask EXPR
umask Sets the umask for the process and returns the old one. If EXPR
is omitted, merely returns current umask.
undef(EXPR)
undef EXPR
undef Undefines the value of EXPR, which must be an lvalue. Use only
on a scalar value, an entire array, or a subroutine name (using
&). (Undef will probably not do what you expect on most
predefined variables or dbm array values.) Always returns the
undefined value. You can omit the EXPR, in which case nothing is
undefined, but you still get an undefined value that you could,
for instance, return from a subroutine. Examples:
undef $foo;
undef $bar{'blurfl'};
undef @ary;
undef %assoc;
undef &mysub;
return (wantarray ? () : undef) if $they_blew_it;
unlink(LIST)
unlink LIST
Deletes a list of files. Returns the number of files
successfully deleted.
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink <*.bak>;
Note: unlink will not delete directories unless you are superuser
and the -U flag is supplied to perl. Even if these conditions
are met, be warned that unlinking a directory can inflict damage
on your filesystem. Use rmdir instead.
unpack(TEMPLATE,EXPR)
Unpack does the reverse of pack: it takes a string representing a
structure and expands it out into an array value, returning the
array value. (In a scalar context, it merely returns the first
value produced.) The TEMPLATE has the same format as in the pack
function. Here's a subroutine that does substring:
sub substr {
local($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ord { unpack("c",$_[0]); }
In addition, you may prefix a field with a %<number> to indicate
that you want a <number>-bit checksum of the items instead of the
items themselves. Default is a 16-bit checksum. For example,
the following computes the same number as the System V sum
program:
while (<>) {
$checksum += unpack("%16C*", $_);
}
$checksum %= 65536;
unshift(ARRAY,LIST)
Does the opposite of a shift. Or the opposite of a push,
depending on how you look at it. Prepends list to the front of
the array, and returns the number of elements in the new array.
unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
utime(LIST)
utime LIST
Changes the access and modification times on each file of a list
of files. The first two elements of the list must be the
NUMERICAL access and modification times, in that order. Returns
the number of files successfully changed. The inode modification
time of each file is set to the current time. Example of a
``touch'' command:
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
values(ASSOC_ARRAY)
values ASSOC_ARRAY
Returns a normal array consisting of all the values of the named
associative array. The values are returned in an apparently
random order, but it is the same order as either the keys() or
each() function would produce on the same array. See also keys()
and each().
vec(EXPR,OFFSET,BITS)
Treats a string as a vector of unsigned integers, and returns the
value of the bitfield specified. May also be assigned to. BITS
must be a power of two from 1 to 32.
Vectors created with vec() can also be manipulated with the
logical operators |, & and ^, which will assume a bit vector
operation is desired when both operands are strings. This
interpretation is not enabled unless there is at least one vec()
in your program, to protect older programs.
To transform a bit vector into a string or array of 0's and 1's,
use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in place of
the *.
wait Waits for a child process to terminate and returns the pid of the
deceased process, or -1 if there are no child processes. The
status is returned in $?.
waitpid(PID,FLAGS)
Waits for a particular child process to terminate and returns the
pid of the deceased process, or -1 if there is no such child
process. The status is returned in $?. If you say
require "sys/wait.h";
...
waitpid(-1,&WNOHANG);
then you can do a non-blocking wait for any process. Non-
blocking wait is only available on machines supporting either the
waitpid (2) or wait4 (2) system calls. However, waiting for a
particular pid with FLAGS of 0 is implemented everywhere. (Perl
emulates the system call by remembering the status values of
processes that have exited but have not been harvested by the
Perl script yet.)
wantarray
Returns true if the context of the currently executing subroutine
is looking for an array value. Returns false if the context is
looking for a scalar.
return wantarray ? () : undef;
warn(LIST)
warn LIST
Produces a message on STDERR just like ``die'', but doesn't exit.
write(FILEHANDLE)
write(EXPR)
write Writes a formatted record (possibly multi-line) to the specified
file, using the format associated with that file. By default the
format for a file is the one having the same name is the
filehandle, but the format for the current output channel (see
select) may be set explicitly by assigning the name of the format
to the $~ variable.
Top of form processing is handled automatically: if there is
insufficient room on the current page for the formatted record,
the page is advanced by writing a form feed, a special top-of-
page format is used to format the new page header, and then the
record is written. By default the top-of-page format is the name
of the filehandle with ``_TOP'' appended, but it may be
dynamicallly set to the format of your choice by assigning the
name to the $^ variable while the filehandle is selected. The
number of lines remaining on the current page is in variable $-,
which can be set to 0 to force a new page.
If FILEHANDLE is unspecified, output goes to the current default
output channel, which starts out as STDOUT but may be changed by
the select operator. If the FILEHANDLE is an EXPR, then the
expression is evaluated and the resulting string is used to look
up the name of the FILEHANDLE at run time. For more on formats,
see the section on formats later on.
Note that write is NOT the opposite of read.
Precedence
Perl operators have the following associativity and precedence:
nonassoc print printf exec system sort reverse
chmod chown kill unlink utime die return
left ,
right = += -= *= etc.
right ?:
nonassoc ..
left ||
left &&
left | ^
left &
nonassoc == != <=> eq ne cmp
nonassoc < > <= >= lt gt le ge
nonassoc chdir exit eval reset sleep rand umask
nonassoc -r -w -x etc.
left << >>
left + - .
left * / % x
left =~ !~
right ! ~ and unary minus
right **
nonassoc ++ --
left `('
As mentioned earlier, if any list operator (print, etc.) or any unary
operator (chdir, etc.) is followed by a left parenthesis as the next
token on the same line, the operator and arguments within parentheses are
taken to be of highest precedence, just like a normal function call.
Examples:
chdir $foo || die; # (chdir $foo) || die
chdir($foo) || die; # (chdir $foo) || die
chdir ($foo) || die; # (chdir $foo) || die
chdir +($foo) || die; # (chdir $foo) || die
but, because * is higher precedence than ||:
chdir $foo * 20; # chdir ($foo * 20)
chdir($foo) * 20; # (chdir $foo) * 20
chdir ($foo) * 20; # (chdir $foo) * 20
chdir +($foo) * 20; # chdir ($foo * 20)
rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
In the absence of parentheses, the precedence of list operators such as
print, sort or chmod is either very high or very low depending on whether
you look at the left side of operator or the right side of it. For
example, in
@ary = (1, 3, sort 4, 2);
print @ary; # prints 1324
the commas on the right of the sort are evaluated before the sort, but
the commas on the left are evaluated after. In other words, list
operators tend to gobble up all the arguments that follow them, and then
act like a simple term with regard to the preceding expression. Note
that you have to be careful with parens:
# These evaluate exit before doing the print:
print($foo, exit); # Obviously not what you want.
print $foo, exit; # Nor is this.
# These do the print before evaluating exit:
(print $foo), exit; # This is what you want.
print($foo), exit; # Or this.
print ($foo), exit; # Or even this.
Also note that
print ($foo & 255) + 1, "\n";
probably doesn't do what you expect at first glance.
Subroutines
A subroutine may be declared as follows:
sub NAME BLOCK
Any arguments passed to the routine come in as array @_, that is ($_[0],
$_[1], ...). The array @_ is a local array, but its values are
references to the actual scalar parameters. The return value of the
subroutine is the value of the last expression evaluated, and can be
either an array value or a scalar value. Alternately, a return statement
may be used to specify the returned value and exit the subroutine. To
create local variables see the local operator.
A subroutine is called using the do operator or the & operator.
Example:
sub MAX {
local($max) = pop(@_);
foreach $foo (@_) {
$max = $foo if $max < $foo;
}
$max;
}
...
$bestday = &MAX($mon,$tue,$wed,$thu,$fri);
Example:
# get a line, combining continuation lines
# that start with whitespace
sub get_line {
$thisline = $lookahead;
line: while ($lookahead = <STDIN>) {
if ($lookahead =~ /^[ \t]/) {
$thisline .= $lookahead;
}
else {
last line;
}
}
$thisline;
}
$lookahead = <STDIN>; # get first line
while ($_ = do get_line()) {
...
}
Use array assignment to a local list to name your formal arguments:
sub maybeset {
local($key, $value) = @_;
$foo{$key} = $value unless $foo{$key};
}
This also has the effect of turning call-by-reference into call-by-value,
since the assignment copies the values.
Subroutines may be called recursively. If a subroutine is called using
the & form, the argument list is optional. If omitted, no @_ array is
set up for the subroutine; the @_ array at the time of the call is
visible to subroutine instead.
do foo(1,2,3); # pass three arguments
&foo(1,2,3); # the same
do foo(); # pass a null list
&foo(); # the same
&foo; # pass no arguments----more efficient
Passing By Reference
Sometimes you don't want to pass the value of an array to a subroutine
but rather the name of it, so that the subroutine can modify the global
copy of it rather than working with a local copy. In perl you can refer
to all the objects of a particular name by prefixing the name with a
star: *foo. When evaluated, it produces a scalar value that represents
all the objects of that name, including any filehandle, format or
subroutine. When assigned to within a local() operation, it causes the
name mentioned to refer to whatever * value was assigned to it. Example:
sub doubleary {
local(*someary) = @_;
foreach $elem (@someary) {
$elem *= 2;
}
}
do doubleary(*foo);
do doubleary(*bar);
Assignment to *name is currently recommended only inside a local(). You
can actually assign to *name anywhere, but the previous referent of *name
may be stranded forever. This may or may not bother you.
Note that scalars are already passed by reference, so you can modify
scalar arguments without using this mechanism by referring explicitly to
the $_[nnn] in question. You can modify all the elements of an array by
passing all the elements as scalars, but you have to use the * mechanism
to push, pop or change the size of an array. The * mechanism will
probably be more efficient in any case.
Since a *name value contains unprintable binary data, if it is used as an
argument in a print, or as a %s argument in a printf or sprintf, it then
has the value '*name', just so it prints out pretty.
Even if you don't want to modify an array, this mechanism is useful for
passing multiple arrays in a single LIST, since normally the LIST
mechanism will merge all the array values so that you can't extract out
the individual arrays.
Regular Expressions
The patterns used in pattern matching are regular expressions such as
those supplied in the Version 8 regexp routines. (In fact, the routines
are derived from Henry Spencer's freely redistributable reimplementation
of the V8 routines.) In addition, \w matches an alphanumeric character
(including ``_'') and \W a nonalphanumeric. Word boundaries may be
matched by \b, and non-boundaries by \B. A whitespace character is
matched by \s, non-whitespace by \S. A numeric character is matched by
\d, non-numeric by \D. You may use \w, \s and \d within character
classes. Also, \n, \r, \f, \t and \NNN have their normal
interpretations. Within character classes \b represents backspace rather
than a word boundary. Alternatives may be separated by |. The
bracketing construct ( ... ) may also be used, in which case \<digit>
matches the digit'th substring. (Outside of the pattern, always use $
instead of \ in front of the digit. The scope of $<digit> (and $`, $&
and $') extends to the end of the enclosing BLOCK or eval string, or to
the next pattern match with subexpressions. The \<digit> notation
sometimes works outside the current pattern, but should not be relied
upon.) You may have as many parentheses as you wish. If you have more
than 9 substrings, the variables $10, $11, ... refer to the corresponding
substring. Within the pattern, \10, \11, etc. refer back to substrings
if there have been at least that many left parens before the
backreference. Otherwise (for backward compatibilty) \10 is the same as
\010, a backspace, and \11 the same as \011, a tab. And so on. (\1
through \9 are always backreferences.)
$+ returns whatever the last bracket match matched. $& returns the
entire matched string. ($0 used to return the same thing, but not any
more.) $` returns everything before the matched string. $' returns
everything after the matched string. Examples:
s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
if (/Time: (..):(..):(..)/) {
$hours = $1;
$minutes = $2;
$seconds = $3;
}
By default, the ^ character is only guaranteed to match at the beginning
of the string, the $ character only at the end (or before the newline at
the end) and perl does certain optimizations with the assumption that the
string contains only one line. The behavior of ^ and $ on embedded
newlines will be inconsistent. You may, however, wish to treat a string
as a multi-line buffer, such that the ^ will match after any newline
within the string, and $ will match before any newline. At the cost of a
little more overhead, you can do this by setting the variable $* to 1.
Setting it back to 0 makes perl revert to its old behavior.
To facilitate multi-line substitutions, the . character never matches a
newline (even when $* is 0). In particular, the following leaves a
newline on the $_ string:
$_ = <STDIN>;
s/.*(some_string).*/$1/;
If the newline is unwanted, try one of
s/.*(some_string).*\n/$1/;
s/.*(some_string)[^\000]*/$1/;
s/.*(some_string)(.|\n)*/$1/;
chop; s/.*(some_string).*/$1/;
/(some_string)/ && ($_ = $1);
Any item of a regular expression may be followed with digits in curly
brackets of the form {n,m}, where n gives the minimum number of times to
match the item and m gives the maximum. The form {n} is equivalent to
{n,n} and matches exactly n times. The form {n,} matches n or more
times. (If a curly bracket occurs in any other context, it is treated as
a regular character.) The * modifier is equivalent to {0,}, the +
modifier to {1,} and the ? modifier to {0,1}. There is no limit to the
size of n or m, but large numbers will chew up more memory.
You will note that all backslashed metacharacters in perl are
alphanumeric, such as \b, \w, \n. Unlike some other regular expression
languages, there are no backslashed symbols that aren't alphanumeric. So
anything that looks like \\, \(, \), \<, \>, \{, or \} is always
interpreted as a literal character, not a metacharacter. This makes it
simple to quote a string that you want to use for a pattern but that you
are afraid might contain metacharacters. Simply quote all the non-
alphanumeric characters:
$pattern =~ s/(\W)/\\$1/g;
Formats
Output record formats for use with the write operator may declared as
follows:
format NAME =
FORMLIST
.
If name is omitted, format ``STDOUT'' is defined. FORMLIST consists of a
sequence of lines, each of which may be of one of three types:
1. A comment.
2. A ``picture'' line giving the format for one output line.
3. An argument line supplying values to plug into a picture line.
Picture lines are printed exactly as they look, except for certain fields
that substitute values into the line. Each picture field starts with
either @ or ^. The @ field (not to be confused with the array marker @)
is the normal case; ^ fields are used to do rudimentary multi-line text
block filling. The length of the field is supplied by padding out the
field with multiple <, >, or | characters to specify, respectively, left
justification, right justification, or centering. As an alternate form
of right justification, you may also use # characters (with an optional
.) to specify a numeric field. (Use of ^ instead of @ causes the field
to be blanked if undefined.) If any of the values supplied for these
fields contains a newline, only the text up to the newline is printed.
The special field @* can be used for printing multi-line values. It
should appear by itself on a line.
The values are specified on the following line, in the same order as the
picture fields. The values should be separated by commas.
Picture fields that begin with ^ rather than @ are treated specially.
The value supplied must be a scalar variable name which contains a text
string. Perl puts as much text as it can into the field, and then chops
off the front of the string so that the next time the variable is
referenced, more of the text can be printed. Normally you would use a
sequence of fields in a vertical stack to print out a block of text. If
you like, you can end the final field with ..., which will appear in the
output if the text was too long to appear in its entirety. You can
change which characters are legal to break on by changing the variable $:
to a list of the desired characters.
Since use of ^ fields can produce variable length records if the text to
be formatted is short, you can suppress blank lines by putting the tilde
(~) character anywhere in the line. (Normally you should put it in the
front if possible, for visibility.) The tilde will be translated to a
space upon output. If you put a second tilde contiguous to the first,
the line will be repeated until all the fields on the line are exhausted.
(If you use a field of the @ variety, the expression you supply had
better not give the same value every time forever!)
Examples:
# a report on the /etc/passwd file
format STDOUT_TOP =
Passwd File
Name Login Office Uid Gid Home
------------------------------------------------------------------
.
format STDOUT =
@<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
$name, $login, $office,$uid,$gid, $home
.
# a report from a bug report form
format STDOUT_TOP =
Bug Reports
@<<<<<<<<<<<<<<<<<<<<<<< @||| @>>>>>>>>>>>>>>>>>>>>>>>
$system, $%, $date
------------------------------------------------------------------
.
format STDOUT =
Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$subject
Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$index, $description
Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$priority, $date, $description
From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$from, $description
Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$programmer, $description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<...
$description
.
It is possible to intermix prints with writes on the same output channel,
but you'll have to handle $- (lines left on the page) yourself.
If you are printing lots of fields that are usually blank, you should
consider using the reset operator between records. Not only is it more
efficient, but it can prevent the bug of adding another field and
forgetting to zero it.
Interprocess Communication
The IPC facilities of perl are built on the Berkeley socket mechanism.
If you don't have sockets, you can ignore this section. The calls have
the same names as the corresponding system calls, but the arguments tend
to differ, for two reasons. First, perl file handles work differently
than C file descriptors. Second, perl already knows the length of its
strings, so you don't need to pass that information. Here is a sample
client (untested):
($them,$port) = @ARGV;
$port = 2345 unless $port;
$them = 'localhost' unless $them;
$SIG{'INT'} = 'dokill';
sub dokill { kill 9,$child if $child; }
require 'sys/socket.ph';
$sockaddr = 'S n a4 x8';
chop($hostname = `hostname`);
($name, $aliases, $proto) = getprotobyname('tcp');
($name, $aliases, $port) = getservbyname($port, 'tcp')
unless $port =~ /^\d+$/;
($name, $aliases, $type, $len, $thisaddr) =
gethostbyname($hostname);
($name, $aliases, $type, $len, $thataddr) = gethostbyname($them);
$this = pack($sockaddr, &AF_INET, 0, $thisaddr);
$that = pack($sockaddr, &AF_INET, $port, $thataddr);
socket(S, &PF_INET, &SOCK_STREAM, $proto) || die "socket: $!";
bind(S, $this) || die "bind: $!";
connect(S, $that) || die "connect: $!";
select(S); $| = 1; select(stdout);
if ($child = fork) {
while (<>) {
print S;
}
sleep 3;
do dokill();
}
else {
while (<S>) {
print;
}
}
And here's a server:
($port) = @ARGV;
$port = 2345 unless $port;
require 'sys/socket.ph';
$sockaddr = 'S n a4 x8';
($name, $aliases, $proto) = getprotobyname('tcp');
($name, $aliases, $port) = getservbyname($port, 'tcp')
unless $port =~ /^\d+$/;
$this = pack($sockaddr, &AF_INET, $port, "\0\0\0\0");
select(NS); $| = 1; select(stdout);
socket(S, &PF_INET, &SOCK_STREAM, $proto) || die "socket: $!";
bind(S, $this) || die "bind: $!";
listen(S, 5) || die "connect: $!";
select(S); $| = 1; select(stdout);
for (;;) {
print "Listening again\n";
($addr = accept(NS,S)) || die $!;
print "accept ok\n";
($af,$port,$inetaddr) = unpack($sockaddr,$addr);
@inetaddr = unpack('C4',$inetaddr);
print "$af $port @inetaddr\n";
while (<NS>) {
print;
print NS;
}
}
Predefined Names
The following names have special meaning to perl. I could have used
alphabetic symbols for some of these, but I didn't want to take the
chance that someone would say reset ``a-zA-Z'' and wipe them all out.
You'll just have to suffer along with these silly symbols. Most of them
have reasonable mnemonics, or analogues in one of the shells.
$_ The default input and pattern-searching space. The following
pairs are equivalent:
while (<>) {... # only equivalent in while!
while ($_ = <>) {...
/^Subject:/
$_ =~ /^Subject:/
y/a-z/A-Z/
$_ =~ y/a-z/A-Z/
chop
chop($_)
(Mnemonic: underline is understood in certain operations.)
$. The current input line number of the last filehandle that was
read. Readonly. Remember that only an explicit close on the
filehandle resets the line number. Since <> never does an
explicit close, line numbers increase across ARGV files (but see
examples under eof). (Mnemonic: many programs use . to mean the
current line number.)
$/ The input record separator, newline by default. Works like awk's
RS variable, including treating blank lines as delimiters if set
to the null string. You may set it to a multicharacter string to
match a multi-character delimiter. Note that setting it to
"\n\n" means something slightly different than setting it to "",
if the file contains consecutive blank lines. Setting it to ""
will treat two or more consecutive blank lines as a single blank
line. Setting it to "\n\n" will blindly assume that the next
input character belongs to the next paragraph, even if it's a
newline. (Mnemonic: / is used to delimit line boundaries when
quoting poetry.)
$, The output field separator for the print operator. Ordinarily
the print operator simply prints out the comma separated fields
you specify. In order to get behavior more like awk, set this
variable as you would set awk's OFS variable to specify what is
printed between fields. (Mnemonic: what is printed when there is
a , in your print statement.)
$ This is like $, except that it applies to array values
interpolated into a double-quoted string (or similar interpreted
string). Default is a space. (Mnemonic: obvious, I think.)
$\ The output record separator for the print operator. Ordinarily
the print operator simply prints out the comma separated fields
you specify, with no trailing newline or record separator
assumed. In order to get behavior more like awk, set this
variable as you would set awk's ORS variable to specify what is
printed at the end of the print. (Mnemonic: you set $\ instead
of adding \n at the end of the print. Also, it's just like /,
but it's what you get ``back'' from perl.)
$# The output format for printed numbers. This variable is a half-
hearted attempt to emulate awk's OFMT variable. There are times,
however, when awk and perl have differing notions of what is in
fact numeric. Also, the initial value is %.20g rather than %.6g,
so you need to set $# explicitly to get awk's value. (Mnemonic:
# is the number sign.)
$% The current page number of the currently selected output channel.
(Mnemonic: % is page number in nroff.)
$= The current page length (printable lines) of the currently
selected output channel. Default is 60. (Mnemonic: = has
horizontal lines.)
$- The number of lines left on the page of the currently selected
output channel. (Mnemonic: lines_on_page - lines_printed.)
$~ The name of the current report format for the currently selected
output channel. Default is name of the filehandle. (Mnemonic:
brother to $^.)
$^ The name of the current top-of-page format for the currently
selected output channel. Default is name of the filehandle with
``_TOP'' appended. (Mnemonic: points to top of page.)
$| If set to nonzero, forces a flush after every write or print on
the currently selected output channel. Default is 0. Note that
STDOUT will typically be line buffered if output is to the
terminal and block buffered otherwise. Setting this variable is
useful primarily when you are outputting to a pipe, such as when
you are running a perl script under rsh and want to see the
output as it's happening. (Mnemonic: when you want your pipes to
be piping hot.)
$$ The process number of the perl running this script. (Mnemonic:
same as shells.)
$? The status returned by the last pipe close, backtick (``) command
or system operator. Note that this is the status word returned
by the wait() system call, so the exit value of the subprocess is
actually ($? >> 8). $? & 255 gives which signal, if any, the
process died from, and whether there was a core dump. (Mnemonic:
similar to sh and ksh.)
$& The string matched by the last successful pattern match (not
counting any matches hidden within a BLOCK or eval enclosed by
the current BLOCK). (Mnemonic: like & in some editors.)
$` The string preceding whatever was matched by the last successful
pattern match (not counting any matches hidden within a BLOCK or
eval enclosed by the current BLOCK). (Mnemonic: ` often precedes
a quoted string.)
$' The string following whatever was matched by the last successful
pattern match (not counting any matches hidden within a BLOCK or
eval enclosed by the current BLOCK). (Mnemonic: ' often follows
a quoted string.) Example:
$_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
$+ The last bracket matched by the last search pattern. This is
useful if you don't know which of a set of alternative patterns
matched. For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.)
$* Set to 1 to do multiline matching within a string, 0 to tell perl
that it can assume that strings contain a single line, for the
purpose of optimizing pattern matches. Pattern matches on
strings containing multiple newlines can produce confusing
results when $* is 0. Default is 0. (Mnemonic: * matches
multiple things.) Note that this variable only influences the
interpretation of ^ and $. A literal newline can be searched for
even when $* == 0.
$0 Contains the name of the file containing the perl script being
executed. Assigning to $0 modifies the argument area that the
ps(1) program sees. (Mnemonic: same as sh and ksh.)
$<digit>
Contains the subpattern from the corresponding set of parentheses
in the last pattern matched, not counting patterns matched in
nested blocks that have been exited already. (Mnemonic: like
\digit.)
$[ The index of the first element in an array, and of the first
character in a substring. Default is 0, but you could set it to
1 to make perl behave more like awk (or Fortran) when
subscripting and when evaluating the index() and substr()
functions. (Mnemonic: [ begins subscripts.)
$] The string printed out when you say ``perl -v''. It can be used
to determine at the beginning of a script whether the perl
interpreter executing the script is in the right range of
versions. If used in a numeric context, returns the version +
patchlevel / 1000. Example:
# see if getc is available
($version,$patchlevel) =
$] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
print STDERR "(No filename completion available.)\n"
if $version * 1000 + $patchlevel < 2016;
or, used numerically,
warn "No checksumming!\n" if $] < 3.019;
(Mnemonic: Is this version of perl in the right bracket?)
$; The subscript separator for multi-dimensional array emulation.
If you refer to an associative array element as
$foo{$a,$b,$c}
it really means
$foo{join($;, $a, $b, $c)}
But don't put
@foo{$a,$b,$c} # a slice----note the @
which means
($foo{$a},$foo{$b},$foo{$c})
Default is "\034", the same as SUBSEP in awk. Note that if your
keys contain binary data there might not be any safe value for
$;. (Mnemonic: comma (the syntactic subscript separator) is a
semi-semicolon. Yeah, I know, it's pretty lame, but $, is
already taken for something more important.)
$! If used in a numeric context, yields the current value of errno,
with all the usual caveats. (This means that you shouldn't
depend on the value of $! to be anything in particular unless
you've gotten a specific error return indicating a system error.)
If used in a string context, yields the corresponding system
error string. You can assign to $! in order to set errno if, for
instance, you want $! to return the string for error n, or you
want to set the exit value for the die operator. (Mnemonic: What
just went bang?)
$@ The perl syntax error message from the last eval command. If
null, the last eval parsed and executed correctly (although the
operations you invoked may have failed in the normal fashion).
(Mnemonic: Where was the syntax error ``at''?)
$< The real uid of this process. (Mnemonic: it's the uid you came
FROM, if you're running setuid.)
$> The effective uid of this process. Example:
$< = $>; # set real uid to the effective uid
($<,$>) = ($>,$<); # swap real and effective uid
(Mnemonic: it's the uid you went TO, if you're running setuid.)
Note: $< and $> can only be swapped on machines supporting
setreuid().
$( The real gid of this process. If you are on a machine that
supports membership in multiple groups simultaneously, gives a
space separated list of groups you are in. The first number is
the one returned by getgid(), and the subsequent ones by
getgroups(), one of which may be the same as the first number.
(Mnemonic: parentheses are used to GROUP things. The real gid is
the group you LEFT, if you're running setgid.)
$) The effective gid of this process. If you are on a machine that
supports membership in multiple groups simultaneously, gives a
space separated list of groups you are in. The first number is
the one returned by getegid(), and the subsequent ones by
getgroups(), one of which may be the same as the first number.
(Mnemonic: parentheses are used to GROUP things. The effective
gid is the group that's RIGHT for you, if you're running setgid.)
Note: $<, $>, $( and $) can only be set on machines that support
the corresponding set[re][ug]id() routine. $( and $) can only be
swapped on machines supporting setregid().
$: The current set of characters after which a string may be broken
to fill continuation fields (starting with ^) in a format.
Default is " \n-", to break on whitespace or hyphens. (Mnemonic:
a ``colon'' in poetry is a part of a line.)
$^D The current value of the debugging flags. (Mnemonic: value of -D
switch.)
$^F The maximum system file descriptor, ordinarily 2. System file
descriptors are passed to subprocesses, while higher file
descriptors are not. During an open, system file descriptors are
preserved even if the open fails. Ordinary file descriptors are
closed before the open is attempted.
$^I The current value of the inplace-edit extension. Use undef to
disable inplace editing. (Mnemonic: value of -i switch.)
$^L What formats output to perform a formfeed. Default is \f.
$^P The internal flag that the debugger clears so that it doesn't
debug itself. You could conceivable disable debugging yourself
by clearing it.
$^T The time at which the script began running, in seconds since the
epoch. The values returned by the -M , -A and -C filetests are
based on this value.
$^W The current value of the warning switch. (Mnemonic: related to
the -w switch.)
$^X The name that Perl itself was executed as, from argv[0].
$ARGV contains the name of the current file when reading from <>.
@ARGV The array ARGV contains the command line arguments intended for
the script. Note that $#ARGV is the generally number of
arguments minus one, since $ARGV[0] is the first argument, NOT
the command name. See $0 for the command name.
@INC The array INC contains the list of places to look for perl
scripts to be evaluated by the ``do EXPR'' command or the
``require'' command. It initially consists of the arguments to
any -I command line switches, followed by the default perl
library, probably ``/usr/local/lib/perl'', followed by ``.'', to
represent the current directory.
%INC The associative array INC contains entries for each filename that
has been included via ``do'' or ``require''. The key is the
filename you specified, and the value is the location of the file
actually found. The ``require'' command uses this array to
determine whether a given file has already been included.
$ENV{expr}
The associative array ENV contains your current environment.
Setting a value in ENV changes the environment for child
processes.
$SIG{expr}
The associative array SIG is used to set signal handlers for
various signals. Example:
sub handler { # 1st argument is signal name
local($sig) = @_;
print "Caught a SIG$sig--shutting down\n";
close(LOG);
exit(0);
}
$SIG{'INT'} = 'handler';
$SIG{'QUIT'} = 'handler';
...
$SIG{'INT'} = 'DEFAULT'; # restore default action
$SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
The SIG array only contains values for the signals actually set
within the perl script.
Packages
Perl provides a mechanism for alternate namespaces to protect packages
from stomping on each others variables. By default, a perl script starts
compiling into the package known as ``main''. By use of the package
declaration, you can switch namespaces. The scope of the package
declaration is from the declaration itself to the end of the enclosing
block (the same scope as the local() operator). Typically it would be
the first declaration in a file to be included by the ``require''
operator. You can switch into a package in more than one place; it
merely influences which symbol table is used by the compiler for the rest
of that block. You can refer to variables and filehandles in other
packages by prefixing the identifier with the package name and a single
quote. If the package name is null, the ``main'' package as assumed.
Only identifiers starting with letters are stored in the packages symbol
table. All other symbols are kept in package ``main''. In addition, the
identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC and SIG are
forced to be in package ``main'', even when used for other purposes than
their built-in one. Note also that, if you have a package called ``m'',
``s'' or ``y'', the you can't use the qualified form of an identifier
since it will be interpreted instead as a pattern match, a substitution
or a translation.
Eval'ed strings are compiled in the package in which the eval was
compiled in. (Assignments to $SIG{}, however, assume the signal handler
specified is in the main package. Qualify the signal handler name if you
wish to have a signal handler in a package.) For an example, examine
perldb.pl in the perl library. It initially switches to the DB package
so that the debugger doesn't interfere with variables in the script you
are trying to debug. At various points, however, it temporarily switches
back to the main package to evaluate various expressions in the context
of the main package.
The symbol table for a package happens to be stored in the associative
array of that name prepended with an underscore. The value in each entry
of the associative array is what you are referring to when you use the
*name notation. In fact, the following have the same effect (in package
main, anyway), though the first is more efficient because it does the
symbol table lookups at compile time:
local(*foo) = *bar;
local($_main{'foo'}) = $_main{'bar'};
You can use this to print out all the variables in a package, for
instance. Here is dumpvar.pl from the perl library:
package dumpvar;
sub main'dumpvar {
($package) = @_;
local(*stab) = eval("*_$package");
while (($key,$val) = each(%stab)) {
{
local(*entry) = $val;
if (defined $entry) {
print "\$$key = '$entry'\n";
}
if (defined @entry) {
print "\@$key = (\n";
foreach $num ($[ .. $#entry) {
print " $num\t'",$entry[$num],"'\n";
}
print ")\n";
}
if ($key ne "_$package" && defined %entry) {
print "\%$key = (\n";
foreach $key (sort keys(%entry)) {
print " $key\t'",$entry{$key},"'\n";
}
print ")\n";
}
}
}
}
Note that, even though the subroutine is compiled in package dumpvar, the
name of the subroutine is qualified so that its name is inserted into
package ``main''.
Style
Each programmer will, of course, have his or her own preferences in
regards to formatting, but there are some general guidelines that will
make your programs easier to read.
1. Just because you CAN do something a particular way doesn't mean that
you SHOULD do it that way. Perl is designed to give you several ways
to do anything, so consider picking the most readable one. For
instance
open(FOO,$foo) || die "Can't open $foo: $!";
is better than
die "Can't open $foo: $!" unless open(FOO,$foo);
because the second way hides the main point of the statement in a
modifier. On the other hand
print "Starting analysis\n" if $verbose;
is better than
$verbose && print "Starting analysis\n";
since the main point isn't whether the user typed -v or not.
Similarly, just because an operator lets you assume default arguments
doesn't mean that you have to make use of the defaults. The defaults
are there for lazy systems programmers writing one-shot programs. If
you want your program to be readable, consider supplying the
argument.
Along the same lines, just because you can omit parentheses in many
places doesn't mean that you ought to:
return print reverse sort num values array;
return print(reverse(sort num (values(%array))));
When in doubt, parenthesize. At the very least it will let some poor
schmuck bounce on the % key in vi.
Even if you aren't in doubt, consider the mental welfare of the
person who has to maintain the code after you, and who will probably
put parens in the wrong place.
2. Don't go through silly contortions to exit a loop at the top or the
bottom, when perl provides the "last" operator so you can exit in the
middle. Just outdent it a little to make it more visible:
line:
for (;;) {
statements;
last line if $foo;
next line if /^#/;
statements;
}
3. Don't be afraid to use loop labels----they're there to enhance
readability as well as to allow multi-level loop breaks. See last
example.
4. For portability, when using features that may not be implemented on
every machine, test the construct in an eval to see if it fails. If
you know what version or patchlevel a particular feature was
implemented, you can test $] to see if it will be there.
5. Choose mnemonic identifiers.
6. Be consistent.
Debugging
If you invoke perl with a -d switch, your script will be run under a
debugging monitor. It will halt before the first executable statement
and ask you for a command, such as:
h Prints out a help message.
T Stack trace.
s Single step. Executes until it reaches the beginning of
another statement.
n Next. Executes over subroutine calls, until it reaches the
beginning of the next statement.
f Finish. Executes statements until it has finished the
current subroutine.
c Continue. Executes until the next breakpoint is reached.
c line Continue to the specified line. Inserts a one-time-only
breakpoint at the specified line.
<CR> Repeat last n or s.
l min+incr List incr+1 lines starting at min. If min is omitted, starts
where last listing left off. If incr is omitted, previous
value of incr is used.
l min-max List lines in the indicated range.
l line List just the indicated line.
l List next window.
- List previous window.
w line List window around line.
l subname List subroutine. If it's a long subroutine it just lists the
beginning. Use ``l'' to list more.
/pattern/ Regular expression search forward for pattern; the final / is
optional.
?pattern? Regular expression search backward for pattern; the final ?
is optional.
L List lines that have breakpoints or actions.
S Lists the names of all subroutines.
t Toggle trace mode on or off.
b line condition
Set a breakpoint. If line is omitted, sets a breakpoint on
the line that is about to be executed. If a condition is
specified, it is evaluated each time the statement is reached
and a breakpoint is taken only if the condition is true.
Breakpoints may only be set on lines that begin an executable
statement.
b subname condition
Set breakpoint at first executable line of subroutine.
d line Delete breakpoint. If line is omitted, deletes the
breakpoint on the line that is about to be executed.
D Delete all breakpoints.
a line command
Set an action for line. A multi-line command may be entered
by backslashing the newlines.
A Delete all line actions.
< command Set an action to happen before every debugger prompt. A
multi-line command may be entered by backslashing the
newlines.
> command Set an action to happen after the prompt when you've just
given a command to return to executing the script. A multi-
line command may be entered by backslashing the newlines.
V package List all variables in package. Default is main package.
! number Redo a debugging command. If number is omitted, redoes the
previous command.
! -number Redo the command that was that many commands ago.
H -number Display last n commands. Only commands longer than one
character are listed. If number is omitted, lists them all.
q or ^D Quit.
command Execute command as a perl statement. A missing semicolon
will be supplied.
p expr Same as ``print DB'OUT expr''. The DB'OUT filehandle is
opened to /dev/tty, regardless of where STDOUT may be
redirected to.
If you want to modify the debugger, copy perldb.pl from the perl library
to your current directory and modify it as necessary. (You'll also have
to put -I. on your command line.) You can do some customization by
setting up a .perldb file which contains initialization code. For
instance, you could make aliases like these:
$DB'alias{'len'} = 's/^len(.*)/p length($1)/';
$DB'alias{'stop'} = 's/^stop (at|in)/b/';
$DB'alias{'.'} =
's/^\./p "\$DB\'sub(\$DB\'line):\t",\$DB\'line[\$DB\'line]/';
Setuid Scripts
Perl is designed to make it easy to write secure setuid and setgid
scripts. Unlike shells, which are based on multiple substitution passes
on each line of the script, perl uses a more conventional evaluation
scheme with fewer hidden ``gotchas''. Additionally, since the language
has more built-in functionality, it has to rely less upon external (and
possibly untrustworthy) programs to accomplish its purposes.
In an unpatched 4.2 or 4.3bsd kernel, setuid scripts are intrinsically
insecure, but this kernel feature can be disabled. If it is, perl can
emulate the setuid and setgid mechanism when it notices the otherwise
useless setuid/gid bits on perl scripts. If the kernel feature isn't
disabled, perl will complain loudly that your setuid script is insecure.
You'll need to either disable the kernel setuid script feature, or put a
C wrapper around the script.
When perl is executing a setuid script, it takes special precautions to
prevent you from falling into any obvious traps. (In some ways, a perl
script is more secure than the corresponding C program.) Any command
line argument, environment variable, or input is marked as ``tainted'',
and may not be used, directly or indirectly, in any command that invokes
a subshell, or in any command that modifies files, directories or
processes. Any variable that is set within an expression that has
previously referenced a tainted value also becomes tainted (even if it is
logically impossible for the tainted value to influence the variable).
For example:
$foo = shift; # $foo is tainted
$bar = $foo,'bar'; # $bar is also tainted
$xxx = <>; # Tainted
$path = $ENV{'PATH'}; # Tainted, but see below
$abc = 'abc'; # Not tainted
system "echo $foo"; # Insecure
system "/bin/echo", $foo; # Secure (doesn't use sh)
system "echo $bar"; # Insecure
system "echo $abc"; # Insecure until PATH set
$ENV{'PATH'} = '/bin:/usr/bin';
$ENV{'IFS'} = '' if $ENV{'IFS'} ne '';
$path = $ENV{'PATH'}; # Not tainted
system "echo $abc"; # Is secure now!
open(FOO,"$foo"); # OK
open(FOO,">$foo"); # Not OK
open(FOO,"echo $foo|"); # Not OK, but...
open(FOO,"-|") || exec 'echo', $foo; # OK
$zzz = `echo $foo`; # Insecure, zzz tainted
unlink $abc,$foo; # Insecure
umask $foo; # Insecure
exec "echo $foo"; # Insecure
exec "echo", $foo; # Secure (doesn't use sh)
exec "sh", '-c', $foo; # Considered secure, alas
The taintedness is associated with each scalar value, so some elements of
an array can be tainted, and others not.
If you try to do something insecure, you will get a fatal error saying
something like ``Insecure dependency'' or ``Insecure PATH''. Note that
you can still write an insecure system call or exec, but only by
explicitly doing something like the last example above. You can also
bypass the tainting mechanism by referencing subpatterns----perl presumes
that if you reference a substring using $1, $2, etc, you knew what you
were doing when you wrote the pattern:
$ARGV[0] =~ /^-P(\w+)$/;
$printer = $1; # Not tainted
This is fairly secure since \w+ doesn't match shell metacharacters. Use
of .+ would have been insecure, but perl doesn't check for that, so you
must be careful with your patterns. This is the ONLY mechanism for
untainting user supplied filenames if you want to do file operations on
them (unless you make $> equal to $<).
It's also possible to get into trouble with other operations that don't
care whether they use tainted values. Make judicious use of the file
tests in dealing with any user-supplied filenames. When possible, do
opens and such after setting $> = $<. Perl doesn't prevent you from
opening tainted filenames for reading, so be careful what you print out.
The tainting mechanism is intended to prevent stupid mistakes, not to
remove the need for thought.
ENVIRONMENT
HOME Used if chdir has no argument.
LOGDIR Used if chdir has no argument and HOME is not set.
PATH Used in executing subprocesses, and in finding the script if
-S is used.
PERLLIB A colon-separated list of directories in which to look for
Perl library files before looking in the standard library and
the current directory.
PERLDB The command used to get the debugger code. If unset, uses
require 'perldb.pl'
Apart from these, perl uses no other environment variables, except to
make them available to the script being executed, and to child processes.
However, scripts running setuid would do well to execute the following
lines before doing anything else, just to keep people honest:
$ENV{'PATH'} = '/bin:/usr/bin'; # or whatever you need
$ENV{'SHELL'} = '/bin/sh' if $ENV{'SHELL'} ne '';
$ENV{'IFS'} = '' if $ENV{'IFS'} ne '';
AUTHOR
Larry Wall <lwall@netlabs.com>
MS-DOS port by Diomidis Spinellis <dds@cc.ic.ac.uk>
FILES
/tmp/perl-eXXXXXX temporary file for -e commands.
SEE ALSO
a2p awk to perl translator
s2p sed to perl translator
DIAGNOSTICS
Compilation errors will tell you the line number of the error, with an
indication of the next token or token type that was to be examined. (In
the case of a script passed to perl via -e switches, each -e is counted
as one line.)
Setuid scripts have additional constraints that can produce error
messages such as ``Insecure dependency''. See the section on setuid
scripts.
TRAPS
Accustomed awk users should take special note of the following:
* Semicolons are required after all simple statements in perl (except
at the end of a block). Newline is not a statement delimiter.
* Curly brackets are required on ifs and whiles.
* Variables begin with $ or @ in perl.
* Arrays index from 0 unless you set $[. Likewise string positions in
substr() and index().
* You have to decide whether your array has numeric or string indices.
* Associative array values do not spring into existence upon mere
reference.
* You have to decide whether you want to use string or numeric
comparisons.
* Reading an input line does not split it for you. You get to split it
yourself to an array. And the split operator has different
arguments.
* The current input line is normally in $_, not $0. It generally does
not have the newline stripped. ($0 is the name of the program
executed.)
* $<digit> does not refer to fields----it refers to substrings matched
by the last match pattern.
* The print statement does not add field and record separators unless
you set $, and $\.
* You must open your files before you print to them.
* The range operator is ``..'', not comma. (The comma operator works
as in C.)
* The match operator is ``=~'', not ``~''. (``~'' is the one's
complement operator, as in C.)
* The exponentiation operator is ``**'', not ``^''. (``^'' is the XOR
operator, as in C.)
* The concatenation operator is ``.'', not the null string. (Using the
null string would render ``/pat/ /pat/'' unparsable, since the third
slash would be interpreted as a division operator----the tokener is
in fact slightly context sensitive for operators like /, ?, and <.
And in fact, . itself can be the beginning of a number.)
* Next, exit and continue work differently.
* The following variables work differently
Awk Perl
ARGC $#ARGV
ARGV[0] $0
FILENAME $ARGV
FNR $. - something
FS (whatever you like)
NF $#Fld, or some such
NR $.
OFMT $#
OFS $,
ORS $\
RLENGTH length($&)
RS $/
RSTART length($`)
SUBSEP $;
* When in doubt, run the awk construct through a2p and see what it
gives you.
Cerebral C programmers should take note of the following:
* Curly brackets are required on ifs and whiles.
* You should use ``elsif'' rather than ``else if''
* Break and continue become last and next, respectively.
* There's no switch statement.
* Variables begin with $ or @ in perl.
* Printf does not implement *.
* Comments begin with #, not /*.
* You can't take the address of anything.
* ARGV must be capitalized.
* The ``system'' calls link, unlink, rename, etc. return nonzero for
success, not 0.
* Signal handlers deal with signal names, not numbers.
Seasoned sed programmers should take note of the following:
* Backreferences in substitutions use $ rather than \.
* The pattern matching metacharacters (, ), and | do not have
backslashes in front.
* The range operator is .. rather than comma.
Sharp shell programmers should take note of the following:
* The backtick operator does variable interpretation without regard to
the presence of single quotes in the command.
* The backtick operator does no translation of the return value, unlike
csh.
* Shells (especially csh) do several levels of substitution on each
command line. Perl does substitution only in certain constructs such
as double quotes, backticks, angle brackets and search patterns.
* Shells interpret scripts a little bit at a time. Perl compiles the
whole program before executing it.
* The arguments are available via @ARGV, not $1, $2, etc.
* The environment is not automatically made available as variables.
ERRATA AND ADDENDA
The Perl book, Programming Perl , has the following omissions and goofs.
On page 5, the examples which read
eval "/usr/bin/perl
should read
eval "exec /usr/bin/perl
On page 195, the equivalent to the System V sum program only works for
very small files. To do larger files, use
undef $/;
$checksum = unpack("%32C*",<>) % 32767;
The descriptions of alarm and sleep refer to signal SIGALARM. These
should refer to SIGALRM.
The -0 switch to set the initial value of $/ was added to Perl after the
book went to press.
The -l switch now does automatic line ending processing.
The qx// construct is now a synonym for backticks.
$0 may now be assigned to set the argument displayed by ps (1).
The new @###.## format was omitted accidentally from the description on
formats.
It wasn't known at press time that s///ee caused multiple evaluations of
the replacement expression. This is to be construed as a feature.
(LIST) x $count now does array replication.
There is now no limit on the number of parentheses in a regular
expression.
In double-quote context, more escapes are supported: \e, \a, \x1b, \c[,
\l, \L, \u, \U, \E. The latter five control up/lower case translation.
The $/ variable may now be set to a multi-character delimiter.
There is now a g modifier on ordinary pattern matching that causes it to
iterate through a string finding multiple matches.
All of the $^X variables are new except for $^T.
The default top-of-form format for FILEHANDLE is now FILEHANDLE_TOP
rather than top.
The eval {} and sort {} constructs were added in version 4.018.
The v and V (little-endian) template options for pack and unpack were
added in 4.019.
BUGS
Perl is at the mercy of your machine's definitions of various operations
such as type casting, atof() and sprintf().
If your stdio requires an seek or eof between reads and writes on a
particular stream, so does perl. (This doesn't apply to sysread() and
syswrite().)
While none of the built-in data types have any arbitrary size limits
(apart from memory size), there are still a few arbitrary limits: a
given identifier may not be longer than 255 characters, and no component
of your PATH may be longer than 255 if you use -S. A regular expression
may not compile to more than 32767 bytes internally.
Perl actually stands for Pathologically Eclectic Rubbish Lister, but
don't tell anyone I said that.